drm/xe: Fix UBSAN splat in add_preempt_fences()

add_preempt_fences() calls dma_resv_reserve_fences() with num_fences ==
0 resulting in the below UBSAN splat. Short circuit add_preempt_fences()
if num_fences == 0.

[   58.652241] ================================================================================
[   58.660736] UBSAN: shift-out-of-bounds in ./include/linux/log2.h:57:13
[   58.667281] shift exponent 64 is too large for 64-bit type 'long unsigned int'
[   58.674539] CPU: 2 PID: 1170 Comm: xe_gpgpu_fill Not tainted 6.6.0-rc3-guc+ #630
[   58.674545] Hardware name: Intel Corporation Tiger Lake Client Platform/TigerLake U DDR4 SODIMM RVP, BIOS TGLSFWI1.R00.3243.A01.2006102133 06/10/2020
[   58.674547] Call Trace:
[   58.674548]  <TASK>
[   58.674550]  dump_stack_lvl+0x92/0xb0
[   58.674555]  __ubsan_handle_shift_out_of_bounds+0x15a/0x300
[   58.674559]  ? rcu_is_watching+0x12/0x60
[   58.674564]  ? software_resume+0x141/0x210
[   58.674575]  ? new_vma+0x44b/0x600 [xe]
[   58.674606]  dma_resv_reserve_fences.cold+0x40/0x66
[   58.674612]  new_vma+0x4b3/0x600 [xe]
[   58.674638]  xe_vm_bind_ioctl+0xffd/0x1e00 [xe]
[   58.674663]  ? __pfx_xe_vm_bind_ioctl+0x10/0x10 [xe]
[   58.674680]  drm_ioctl_kernel+0xc1/0x170
[   58.674686]  ? __pfx_xe_vm_bind_ioctl+0x10/0x10 [xe]
[   58.674703]  drm_ioctl+0x247/0x4c0
[   58.674709]  ? find_held_lock+0x2b/0x80
[   58.674716]  __x64_sys_ioctl+0x8c/0xb0
[   58.674720]  do_syscall_64+0x3c/0x90
[   58.674723]  entry_SYSCALL_64_after_hwframe+0x6e/0xd8
[   58.674727] RIP: 0033:0x7fce4bd1aaff
[   58.674730] Code: 00 48 89 44 24 18 31 c0 48 8d 44 24 60 c7 04 24 10 00 00 00 48 89 44 24 08 48 8d 44 24 20 48 89 44 24 10 b8 10 00 00 00 0f 05 <41> 89 c0 3d 00 f0 ff ff 77 1f 48 8b 44 24 18 64 48 2b 04 25 28 00
[   58.674731] RSP: 002b:00007ffc57434050 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
[   58.674734] RAX: ffffffffffffffda RBX: 00007ffc574340e0 RCX: 00007fce4bd1aaff
[   58.674736] RDX: 00007ffc574340e0 RSI: 0000000040886445 RDI: 0000000000000003
[   58.674737] RBP: 0000000040886445 R08: 0000000000000002 R09: 00007ffc574341b0
[   58.674739] R10: 000055de43eb3780 R11: 0000000000000246 R12: 00007ffc574340e0
[   58.674740] R13: 0000000000000003 R14: 00007ffc574341b0 R15: 0000000000000001
[   58.674747]  </TASK>
[   58.674748] ================================================================================

Fixes: dd08ebf6 ("drm/xe: Introduce a new DRM driver for Intel GPUs")
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://lore.kernel.org/r/20231215230203.719244-1-matthew.brost@intel.comSigned-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

drivers/gpu/drm/xe/xe_vm.c

@@ -279,6 +279,9 @@ static int add_preempt_fences(struct xe_vm *vm, struct xe_bo *bo)
 	struct xe_exec_queue *q;
 	int err;
 
+	if (!vm->preempt.num_exec_queues)
+		return 0;
+
 	err = xe_bo_lock(bo, true);
 	if (err)
 		return err;