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