drm/radeon: Use drm_malloc_ab instead of kmalloc_array

Should avoid kmalloc failures due to large number of array entries.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=81991


Reviewed-by: Dave Airlie <airlied@redhat.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>
Cc: stable@vger.kernel.org