drm/udl: Add register constants for color depth

Add the register constants for setting the color depth. The driver
only uses 16bpp. No functional changes.
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221006095355.23579-14-tzimmermann@suse.de

drivers/gpu/drm/udl/udl_modeset.c

@@ -56,7 +56,7 @@ static char *udl_set_blank_mode(char *buf, u8 mode)
 
 static char *udl_set_color_depth(char *buf, u8 selection)
 {
-	return udl_set_register(buf, 0x00, selection);
+	return udl_set_register(buf, UDL_REG_COLORDEPTH, selection);
 }
 
 static char *udl_set_base16bpp(char *wrptr, u32 base)

drivers/gpu/drm/udl/udl_proto.h

@@ -3,7 +3,10 @@
 #ifndef UDL_PROTO_H
 #define UDL_PROTO_H
 
+/* Color depth */
+#define UDL_REG_COLORDEPTH		0x00
 #define UDL_COLORDEPTH_16BPP		0
+#define UDL_COLORDEPTH_24BPP		1
 
 /* Display-mode settings */
 #define UDL_REG_XDISPLAYSTART		0x01