drm/{i915, xe}/frontbuffer: move frontbuffer handling to parent interface

drm/{i915, xe}/frontbuffer: move frontbuffer handling to parent interface

Move the get/put/ref/flush_for_display calls to the display parent
interface.

For i915, move the hooks next to the other i915 core frontbuffer code in
i915_gem_object_frontbuffer.c. For xe, add new file xe_frontbuffer.c for
the same.

Note: The intel_frontbuffer_flush() calls from
i915_gem_object_frontbuffer.c will partially route back to i915 core via
the parent interface. This is less than stellar.

Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Link: https://patch.msgid.link/f69b967ed82bbcfd60ffa77ba197b26a1399f09f.1772475391.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>