OMAP: DSS2: Add new FEAT definitions for features missing from OMAP2

OMAP2 doesn't have CPR, PRELOAD nor FIR_COEF_V registers. Add new
feature definitions for those, and check the feature before accessing
those registers.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
diff --git a/drivers/video/omap2/dss/overlay.c b/drivers/video/omap2/dss/overlay.c
index 0f08025..cfbfc57 100644
--- a/drivers/video/omap2/dss/overlay.c
+++ b/drivers/video/omap2/dss/overlay.c
@@ -238,6 +238,9 @@
 	u8 alpha;
 	struct omap_overlay_info info;
 
+	if (!dss_has_feature(FEAT_GLOBAL_ALPHA))
+		return -ENODEV;
+
 	r = kstrtou8(buf, 0, &alpha);
 	if (r)
 		return r;