drm/cma-helper: simplify setup for drivers with ->dirty callbacks

If we store the fb funcs pointer, we can remove a bit of boilerplate.
Also remove the _fbdev_ in the example code, since the fb_funcs->dirty
callback has nothing to do with fbdev. It's a KMS feature, only
used by the fbdev deferred_io support to implement flushing/upload.

Cc: Noralf Trønnes <noralf@tronnes.org>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
[danvet: Move the misplaced kerneldoc change from a later patch to
this one here.]
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1483044517-5770-11-git-send-email-daniel.vetter@ffwll.ch