drm: Forward-declare struct drm_format_info in drm_framebuffer.h

drm_framebuffer.h makes use of a pointer to struct drm_format_info but
doesn't include drm_fourcc.h (neither directly nor indirectly).
Forward-declare the structure.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>