drm/fb-helper: don't preserve fb_ops across deferred IO use

Deferred IO now preserves the fb_ops.

v2: Remove the no-op vfree, drop a local var  (Noralf)

Cc: Noralf Trønnes <noralf@tronnes.org>
Cc: dri-devel@lists.freedesktop.org
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1eae0b23d4724d5702b886b6a061ec8219eb9284.1575022735.git.jani.nikula@intel.com