amdgpu: Don't use DRM_ERROR when failing to allocate a BO

This can be triggered by userspace, e.g. trying to allocate too large a
BO, so it shouldn't log anything by default.

Callers need to handle failure anyway.

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