drm/radeon: split evergreen_cs_check_reg

evergreen_cs_check_reg() is a large function and gcc doesn't want to
inline it. It has a quick check for reg_safe_bm[] to see if register
needs special handling, which often results in early exit. However
because the function is large, it has a long prologue/epilogue to
save/restore all the callee-save registers which according to perf is
taking significant amount of time. To avoid this, we can reuse
evergreen_is_safe_reg() to do the early check directly in register loop.

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