drm: omapdrm: fb: Use format information provided by the DRM core

The driver stores in a custom structure named format several pieces of
information about the format that are available in the DRM core. Remove
them and get the information from the DRM core instead.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>