drm/gem-fb-helper: Use actual bpp for size calculations

The AFBC helpers derive the number of bits per pixel from the deprecated
drm_format_info.cpp[] field, which does not take into account block
sizes.

Fix this by using the actual number of bits per pixel instead.
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/b1d938d903157627c4ac101df92a1f3bf8ebe83d.1657294931.git.geert@linux-m68k.org

drivers/gpu/drm/drm_gem_framebuffer_helper.c

@@ -490,6 +490,8 @@ void drm_gem_fb_end_cpu_access(struct drm_framebuffer *fb, enum dma_data_directi
 }
 EXPORT_SYMBOL(drm_gem_fb_end_cpu_access);
 
+// TODO Drop this function and replace by drm_format_info_bpp() once all
+// DRM_FORMAT_* provide proper block info in drivers/gpu/drm/drm_fourcc.c
 static __u32 drm_gem_afbc_get_bpp(struct drm_device *dev,
 				  const struct drm_mode_fb_cmd2 *mode_cmd)
 {
@@ -497,11 +499,6 @@ static __u32 drm_gem_afbc_get_bpp(struct drm_device *dev,
 
 	info = drm_get_format_info(dev, mode_cmd);
 
-	/* use whatever a driver has set */
-	if (info->cpp[0])
-		return info->cpp[0] * 8;
-
-	/* guess otherwise */
 	switch (info->format) {
 	case DRM_FORMAT_YUV420_8BIT:
 		return 12;
@@ -510,11 +507,8 @@ static __u32 drm_gem_afbc_get_bpp(struct drm_device *dev,
 	case DRM_FORMAT_VUY101010:
 		return 30;
 	default:
-		break;
+		return drm_format_info_bpp(info, 0);
 	}
-
-	/* all attempts failed */
-	return 0;
 }
 
 static int drm_gem_afbc_min_size(struct drm_device *dev,