nouveau: Skip unvailable ttm page entries

Starting with commit f295c8cf
("drm/nouveau: fix dma syncing warning with debugging on.")
the following oops occures:

   BUG: kernel NULL pointer dereference, address: 0000000000000000
   #PF: supervisor read access in kernel mode
   #PF: error_code(0x0000) - not-present page
   PGD 0 P4D 0
   Oops: 0000 [#1] PREEMPT SMP PTI
   CPU: 6 PID: 1013 Comm: Xorg.bin Tainted: G E     5.11.0-desktop-rc0+ #2
   Hardware name: Acer Aspire VN7-593G/Pluto_KLS, BIOS V1.11 08/01/2018
   RIP: 0010:nouveau_bo_sync_for_device+0x40/0xb0 [nouveau]
   Call Trace:
    nouveau_bo_validate+0x5d/0x80 [nouveau]
    nouveau_gem_ioctl_pushbuf+0x662/0x1120 [nouveau]
    ? nouveau_gem_ioctl_new+0xf0/0xf0 [nouveau]
    drm_ioctl_kernel+0xa6/0xf0 [drm]
    drm_ioctl+0x1f4/0x3a0 [drm]
    ? nouveau_gem_ioctl_new+0xf0/0xf0 [nouveau]
    nouveau_drm_ioctl+0x50/0xa0 [nouveau]
    __x64_sys_ioctl+0x7e/0xb0
    do_syscall_64+0x33/0x80
    entry_SYSCALL_64_after_hwframe+0x44/0xae
   ---[ end trace ccfb1e7f4064374f ]---
   RIP: 0010:nouveau_bo_sync_for_device+0x40/0xb0 [nouveau]

The underlying problem is not introduced by the commit, yet it uncovered the
underlying issue. The cited commit relies on valid pages. This is not given for
due to some bugs. For now, just warn and work around the issue by just ignoring
the bad ttm objects.
Below is some debug info gathered while debugging this issue:

nouveau 0000:01:00.0: DRM: ttm_dma->num_pages: 2048
nouveau 0000:01:00.0: DRM: ttm_dma->pages is NULL
nouveau 0000:01:00.0: DRM: ttm_dma: 00000000e96058e7
nouveau 0000:01:00.0: DRM: ttm_dma->page_flags:
nouveau 0000:01:00.0: DRM: ttm_dma:   Populated: 1
nouveau 0000:01:00.0: DRM: ttm_dma:   No Retry: 0
nouveau 0000:01:00.0: DRM: ttm_dma:   SG: 256
nouveau 0000:01:00.0: DRM: ttm_dma:   Zero Alloc: 0
nouveau 0000:01:00.0: DRM: ttm_dma:   Swapped: 0
Signed-off-by: Tobias Klausmann <tobias.klausmann@freenet.de>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210313222159.3346-1-tobias.klausmann@freenet.de

drivers/gpu/drm/nouveau/nouveau_bo.c

@@ -551,6 +551,10 @@ nouveau_bo_sync_for_device(struct nouveau_bo *nvbo)
 
 	if (!ttm_dma)
 		return;
+	if (!ttm_dma->pages) {
+		NV_DEBUG(drm, "ttm_dma 0x%p: pages NULL\n", ttm_dma);
+		return;
+	}
 
 	/* Don't waste time looping if the object is coherent */
 	if (nvbo->force_coherent)
@@ -583,6 +587,10 @@ nouveau_bo_sync_for_cpu(struct nouveau_bo *nvbo)
 
 	if (!ttm_dma)
 		return;
+	if (!ttm_dma->pages) {
+		NV_DEBUG(drm, "ttm_dma 0x%p: pages NULL\n", ttm_dma);
+		return;
+	}
 
 	/* Don't waste time looping if the object is coherent */
 	if (nvbo->force_coherent)