drm/radeon: add WAIT_UNTIL to evergreen VM safe reg list

It's used in a recent mesa commit:
http://cgit.freedesktop.org/mesa/mesa/commit/?id=24b1206ab2dcd506aaac3ef656aebc8bc20cd27a
and there may be some other cases in the future where it's required.
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Jerome Glisse <jglisse@redhat.com>
Cc: stable@vger.kernel.org

drivers/gpu/drm/radeon/evergreen_cs.c

@@ -3316,6 +3316,7 @@ static bool evergreen_vm_reg_valid(u32 reg)
 
 	/* check config regs */
 	switch (reg) {
+	case WAIT_UNTIL:
 	case GRBM_GFX_INDEX:
 	case CP_STRMOUT_CNTL:
 	case CP_COHER_CNTL: