drm/sysfb: Find screen_info format with helpers

Convert drm_sysfb_get_format_si() to lookup the screen_info color
format as struct pixel_format with screen_info_pixel_format(). Then
search the list of given formats for the screen_info format with
pixel_format_equal().

Replaces custom code with helpers. The pixel-compare helper
pixel_format_equal() also handles indexed color formats. Prepares
for sysfb drivers to support color palettes.

Signed-off-by: Thomas Zimmermann <[e-mail]>
Reviewed-by: Javier Martinez Canillas <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

drivers/gpu/drm/sysfb/drm_sysfb_screen_info.c

···
 						      const struct screen_info *si)
 {
 	const struct drm_format_info *format = NULL;
-	u32 bits_per_pixel;
+	struct pixel_format pixel;
 	size_t i;
+	int ret;
 
-	bits_per_pixel = __screen_info_lfb_bits_per_pixel(si);
+	ret = screen_info_pixel_format(si, &pixel);
+	if (ret)
+		return NULL;
 
 	for (i = 0; i < nformats; ++i) {
-		const struct pixel_format *f = &formats[i].pixel;
+		const struct drm_sysfb_format *f = &formats[i];
 
-		if (bits_per_pixel == f->bits_per_pixel &&
-		    si->red_size == f->red.length &&
-		    si->red_pos == f->red.offset &&
-		    si->green_size == f->green.length &&
-		    si->green_pos == f->green.offset &&
-		    si->blue_size == f->blue.length &&
-		    si->blue_pos == f->blue.offset) {
-			format = drm_format_info(formats[i].fourcc);
+		if (pixel_format_equal(&pixel, &f->pixel)) {
+			format = drm_format_info(f->fourcc);
 			break;
 		}
 	}