drm/radeon/kms: allow FG_ALPHA_VALUE on r5xx

This is a CS checker fix. I need this for FP16 alpha-test.
Signed-off-by: NMarek Olšák <maraeo@gmail.com>
Signed-off-by: NDave Airlie <airlied@redhat.com>

drivers/gpu/drm/radeon/reg_srcs/rv515

@@ -316,6 +316,7 @@ rv515 0x6d40
 0x4BD0 FG_FOG_COLOR_B
 0x4BD4 FG_ALPHA_FUNC
 0x4BD8 FG_DEPTH_SRC
+0x4BE0 FG_ALPHA_VALUE
 0x4C00 US_ALU_CONST_R_0
 0x4C04 US_ALU_CONST_G_0
 0x4C08 US_ALU_CONST_B_0