drm/i915/display: Move releasing gem object away from fb tracking

As a preparation for Xe we want to remove all i915_gem_object details away
from frontbuffer tacking code. Due to this move releasing gem object
reference to i915_gem_object_set_frontbuffer.

Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Reviewed-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231012072158.4115795-2-jouni.hogander@intel.com