drm/radeon/kms: allow rendering while no colorbuffer is set on r300

Because hardware cannot disable all colorbuffers directly to do depth-only
rendering, a user should:
- disable reading from a colorbuffer in blending
- disable fastfill
- set the color channel mask to 0 to prevent writing to a colorbuffer

Signed-off-by: Dave Airlie <airlied@redhat.com>
diff --git a/drivers/gpu/drm/radeon/r100_track.h b/drivers/gpu/drm/radeon/r100_track.h
index 7188c37..b27a699 100644
--- a/drivers/gpu/drm/radeon/r100_track.h
+++ b/drivers/gpu/drm/radeon/r100_track.h
@@ -67,13 +67,15 @@
 	unsigned			immd_dwords;
 	unsigned			num_arrays;
 	unsigned			max_indx;
+	unsigned			color_channel_mask;
 	struct r100_cs_track_array	arrays[11];
 	struct r100_cs_track_cb 	cb[R300_MAX_CB];
 	struct r100_cs_track_cb 	zb;
 	struct r100_cs_track_texture	textures[R300_TRACK_MAX_TEXTURE];
 	bool				z_enabled;
 	bool                            separate_cube;
-
+	bool				fastfill;
+	bool				blend_read_enable;
 };
 
 int r100_cs_track_check(struct radeon_device *rdev, struct r100_cs_track *track);