drm/framebuffer: drm_framebuffer_read_refcount() constify argument

Constify argument so functions calling into this take a const argument.
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171107191348.17555-3-noralf@tronnes.org

include/drm/drm_framebuffer.h

@@ -263,7 +263,7 @@ static inline void drm_framebuffer_unreference(struct drm_framebuffer *fb)
  *
  * This functions returns the framebuffer's reference count.
  */
-static inline uint32_t drm_framebuffer_read_refcount(struct drm_framebuffer *fb)
+static inline uint32_t drm_framebuffer_read_refcount(const struct drm_framebuffer *fb)
 {
 	return kref_read(&fb->base.refcount);
 }