drm/fb_cma_helper: Replace drm_format_info() with fb->format

Get the format information via the neat fb->format pointer rather than
doing a linear search over all the format info structures.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1479498793-31021-27-git-send-email-ville.syrjala@linux.intel.com