This patch fixes the case when buffer funcs is empty and bo evict is
executing. It must double check buffer funcs, otherwise, a NULL
pointer dereference kernel panic will be encountered.

 BUG: unable to handle kernel NULL pointer dereference at 00000000000001a4
 IP: [<ffffffffa067b6cd>] amdgpu_evict_flags+0x3d/0xf0 [amdgpu]
 PGD 0

 Oops: 0000 [#1] SMP
 Modules linked in: amdgpu(OE) ttm drm_kms_helper drm i2c_algo_bit fb_sys_fops syscopyarea sysfillrect sysimgblt fmem(OE) physmem_drv(OE) rpcsec_gss_krb5 nfsv4 nfs fscache intel_rapl x86_pkg_temp_thermal intel_powerclamp snd_hda_codec_realtek snd_hda_codec_hdmi snd_hda_codec_generic kvm_intel snd_hda_intel snd_hda_codec kvm snd_hda_core joydev eeepc_wmi asus_wmi sparse_keymap snd_hwdep snd_pcm irqbypass crct10dif_pclmul snd_seq_midi snd_seq_midi_event snd_rawmidi snd_seq crc32_pclmul snd_seq_device ghash_clmulni_intel aesni_intel aes_x86_64 snd_timer lrw gf128mul mei_me snd glue_helper ablk_helper cryptd tpm_infineon mei lpc_ich serio_raw soundcore shpchp mac_hid nfsd auth_rpcgss nfs_acl lockd grace coretemp sunrpc parport_pc ppdev lp parport autofs4 hid_generic mxm_wmi r8169 usbhid ahci
  psmouse libahci nvme mii hid nvme_core wmi video
 CPU: 3 PID: 1627 Comm: kworker/u8:17 Tainted: G           OE   4.9.0-custom #1
 Hardware name: ASUS All Series/Z87-A, BIOS 1802 01/28/2014
 Workqueue: events_unbound async_run_entry_fn
 task: ffff88021e7057c0 task.stack: ffffc9000262c000
 RIP: 0010:[<ffffffffa067b6cd>]  [<ffffffffa067b6cd>] amdgpu_evict_flags+0x3d/0xf0 [amdgpu]
 RSP: 0018:ffffc9000262fb30  EFLAGS: 00010246
 RAX: 0000000000000000 RBX: ffff88021e8a5858 RCX: 0000000000000000
 RDX: 0000000000000001 RSI: ffffc9000262fb58 RDI: ffff88021e8a5800
 RBP: ffffc9000262fb48 R08: 0000000000000000 R09: ffff88021e8a5814
 R10: 000000001def8f01 R11: ffff88021def8c80 R12: ffffc9000262fb58
 R13: ffff88021d2b1990 R14: 0000000000000000 R15: ffff88021e8a5858
 FS:  0000000000000000(0000) GS:ffff88022ed80000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 00000000000001a4 CR3: 0000000001c07000 CR4: 00000000001406e0
 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
 DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Signed-off-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Ken Wang <Qingqing.Wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>