drm/amdgpu: Remove unnecessary NULL check

container_of cannot return NULL, so it is unnecessary to check for
NULL after gem_to_amdgpu_bo, which is just a container_of call

Signed-off-by: Kent Russell <kent.russell@amd.com>
Reviewed-by: Felix Kuehling <felix.kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>