drm/omapdrm: Nuke dummy fb->dirty callback

It's an optional hook.

Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1464630800-30786-23-git-send-email-daniel.vetter@ffwll.ch