drm/i915/overlay: convert from struct intel_frontbuffer to i915_frontbuffer

drm/i915/overlay: convert from struct intel_frontbuffer to i915_frontbuffer

The intel_frontbuffer_get() and intel_frontbuffer_put() calls are routed
through intel_frontbuffer.c to i915_gem_object_frontbuffer.c. We might
as well call the functions directly, instead of going through display
code. This would only get worse with get/put being moved to the parent
interface.

To make this easier, convert overlay code from struct intel_frontbuffer
to struct i915_frontbuffer, and add a
i915_gem_object_frontbuffer_track() wrapper for clarity.

Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Link: https://patch.msgid.link/829b304a6451e80fbce554bdc7788077245e803a.1772475391.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>