drm/radeon: simplify register checker

To avoid having to distinguish between CAYMAN or older on every register
check, place a pointer in evergreen_cs_track and use it unconditionally.
Also make use of the fact that both reg_safe_bm[] arrays are of the same
length to remove another CAYMAN check.

Reviewed-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>