drm/i915: Clean up skl+ plane alpha bits

Convert a few more skl+ plane registers to REG_BIT() & co.
Somehow thse were missed during the earlier cleanup.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230314130255.23273-8-ville.syrjala@linux.intel.comReviewed-by: Jouni Högander <jouni.hogander@intel.com>

drivers/gpu/drm/i915/i915_reg.h

@@ -4591,10 +4591,11 @@
 #define _PLANE_KEYVAL_2_A			0x70294
 #define _PLANE_KEYMSK_1_A			0x70198
 #define _PLANE_KEYMSK_2_A			0x70298
-#define  PLANE_KEYMSK_ALPHA_ENABLE		(1 << 31)
+#define   PLANE_KEYMSK_ALPHA_ENABLE		REG_BIT(31)
 #define _PLANE_KEYMAX_1_A			0x701a0
 #define _PLANE_KEYMAX_2_A			0x702a0
-#define  PLANE_KEYMAX_ALPHA(a)			((a) << 24)
+#define   PLANE_KEYMAX_ALPHA_MASK		REG_GENMASK(31, 24)
+#define   PLANE_KEYMAX_ALPHA(a)			REG_FIELD_PREP(PLANE_KEYMAX_ALPHA_MASK, (a))
 #define _PLANE_SURFLIVE_1_A			0x701ac
 #define _PLANE_SURFLIVE_2_A			0x702ac
 #define _PLANE_CC_VAL_1_A			0x701b4