drm/radeon: don't evict if not initialized

TTM_PL_VRAM may not initialized at all when calling
radeon_bo_evict_vram(). We need to check before doing eviction.

[    2.160837] BUG: kernel NULL pointer dereference, address: 0000000000000020
[    2.161212] #PF: supervisor read access in kernel mode
[    2.161490] #PF: error_code(0x0000) - not-present page
[    2.161767] PGD 0 P4D 0
[    2.163088] RIP: 0010:ttm_resource_manager_evict_all+0x70/0x1c0 [ttm]
[    2.168506] Call Trace:
[    2.168641]  radeon_bo_evict_vram+0x1c/0x20 [radeon]
[    2.168936]  radeon_device_fini+0x28/0xf9 [radeon]
[    2.169224]  radeon_driver_unload_kms+0x44/0xa0 [radeon]
[    2.169534]  radeon_driver_load_kms+0x174/0x210 [radeon]
[    2.169843]  drm_dev_register+0xd9/0x1c0 [drm]
[    2.170104]  radeon_pci_probe+0x117/0x1a0 [radeon]

Reviewed-by: Christian König <christian.koenig@amd.com>
Suggested-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Tong Zhang <ztong0001@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>