drm/repaper: Use format helper for xrgb8888 to monochrome conversion

There is now a drm_fb_xrgb8888_to_mono_reversed() helper function to do
format conversion from XRGB8888 to reversed monochrome.

Use that helper and remove the open coded version in the repaper driver.
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Tested-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220223193735.213185-1-javierm@redhat.com

drivers/gpu/drm/tiny/repaper.c

@@ -508,26 +508,6 @@ static void repaper_get_temperature(struct repaper_epd *epd)
 	epd->factored_stage_time = epd->stage_time * factor10x / 10;
 }
 
-static void repaper_gray8_to_mono_reversed(u8 *buf, u32 width, u32 height)
-{
-	u8 *gray8 = buf, *mono = buf;
-	int y, xb, i;
-
-	for (y = 0; y < height; y++)
-		for (xb = 0; xb < width / 8; xb++) {
-			u8 byte = 0x00;
-
-			for (i = 0; i < 8; i++) {
-				int x = xb * 8 + i;
-
-				byte >>= 1;
-				if (gray8[y * width + x] >> 7)
-					byte |= BIT(7);
-			}
-			*mono++ = byte;
-		}
-}
-
 static int repaper_fb_dirty(struct drm_framebuffer *fb)
 {
 	struct drm_gem_cma_object *cma_obj = drm_fb_cma_get_gem_obj(fb, 0);
@@ -560,12 +540,10 @@ static int repaper_fb_dirty(struct drm_framebuffer *fb)
 	if (ret)
 		goto out_free;
 
-	drm_fb_xrgb8888_to_gray8(buf, 0, cma_obj->vaddr, fb, &clip);
+	drm_fb_xrgb8888_to_mono_reversed(buf, 0, cma_obj->vaddr, fb, &clip);
 
 	drm_gem_fb_end_cpu_access(fb, DMA_FROM_DEVICE);
 
-	repaper_gray8_to_mono_reversed(buf, fb->width, fb->height);
-
 	if (epd->partial) {
 		repaper_frame_data_repeat(epd, buf, epd->current_frame,
 					  REPAPER_NORMAL);