drm/fourcc: Add drm_format_info_bpp() helper

Add a helper to retrieve the actual number of bits per pixel for a
plane, taking into account the number of characters and pixels per
block for tiled formats.

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/1cae5ebc28513ec1c91c66b00647ce3ca23bfba7.1657294931.git.geert@linux-m68k.org