drm/radeon: Use drm_calloc_ab for CS relocs

The number of relocs is passed in by userspace and can be large. It has
been observed to cause kcalloc failures in the wild.

Cc: stable@vger.kernel.org
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>