drm/amdgpu: fix error handling in amdgpu_bo_create_restricted

Manually checking why a function could fail is not a good idea
if you can just check the functions return code.

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