drm/radeon: add WAIT_UNTIL to the non-VM safe regs list for cayman/TN

Newer versions of mesa emit this.
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org

drivers/gpu/drm/radeon/reg_srcs/cayman

 cayman 0x9400
 0x0000802C GRBM_GFX_INDEX
+0x00008040 WAIT_UNTIL
 0x000084FC CP_STRMOUT_CNTL
 0x000085F0 CP_COHER_CNTL
 0x000085F4 CP_COHER_SIZE