drm/i915/overlay: Extract i915_overlay_setup()

drm/i915/overlay: Extract i915_overlay_setup()

Pull the gem/gt related bits of the overlay setup into
a separate function (i915_overlay_setup()) that will eventually
move to the i915 side of the parent vs. display driver split.

For now we'll also have to pass in the overlay struct, but
that will disappear once the i915 vs. display split is completed.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/20260226100738.29997-10-ville.syrjala@linux.intel.com