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>

drivers/gpu/drm/drm_modeset_helper.c

@@ -91,6 +91,7 @@ void drm_helper_mode_fill_fb_struct(struct drm_device *dev,
 	}
 
 	fb->dev = dev;
+	fb->format = info;
 	fb->width = mode_cmd->width;
 	fb->height = mode_cmd->height;
 	for (i = 0; i < 4; i++) {

include/drm/drm_framebuffer.h

@@ -121,6 +121,10 @@ struct drm_framebuffer {
 	 * @base: base modeset object structure, contains the reference count.
 	 */
 	struct drm_mode_object base;
+	/**
+	 * @format: framebuffer format information
+	 */
+	const struct drm_format_info *format;
 	/**
 	 * @funcs: framebuffer vfunc table
 	 */