drm: Store a pointer to drm_format_info under drm_framebuffer

drm: Store a pointer to drm_format_info under drm_framebuffer

To avoid having to look up the format information struct every time,
let's just store a pointer to it under drm_framebuffer.

v2: Don't populate the fb->format pointer in drm_framebuffer_init().
    instead we'll treat a NULL format as an error later

Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com> (v1)
Link: http://patchwork.freedesktop.org/patch/msgid/1479498793-31021-20-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>