drm/i915: further deduplicate intel_find_initial_plane_obj()

drm/i915: further deduplicate intel_find_initial_plane_obj()

Move intel_reuse_initial_plane_obj() into common display code, and split
the ->find_obj hook into ->alloc_obj and ->setup hooks.

Return the struct drm_gem_object from ->alloc_obj in preparation for
moving more things to display.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/c71011dbb11afaa5c4da30aa2627833374300d63.1765812266.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>