drm/gem-fb-helper: drm_gem_fbdev_fb_create() make funcs optional

Make the drm_framebuffer_funcs argument optional for drivers that
don't need to set the dirty callback.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20171115142001.45358-2-noralf@tronnes.org