Another attempt at restoring the fbdev mode. Changes from v1: - Cursors and sprites are disabled permanently (well, until someone explicitly re-enabls them). This was actually already the case for the old video overlay. - Since the disabling is now permanent, all extra planes can be disabled before the crtc modeset/set_base is performed so there should be no extra blinking of the planes I still left the new fbdev_restore_mode hook in place because I was too lazy to figure out if I could just disable planes/cursors from drm_fb_helper directly (i915 would be fine, but other drivers could have issues with it), and even with i915 we still have to disable the old video overlay, which is driven via some custom API, and hence can't be disabled from the common code.
