Userspace should set the first 4 bits of drm_radeon_cs_reloc::flags to
a number from 0 to 15. The higher the number, the higher the priority,
which means a buffer with a higher number will be validated sooner.

The old behavior is preserved: Buffers used for write are prioritized over
read-only buffers if the userspace doesn't set the number.

v2: add buffers to buckets directly, then concatenate them
v3: use a stable sort
Signed-off-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>