drm/i915: move display/intel_plane_initial.c to i915_initial_plane.c
intel_plane_initial.c is i915 specific. Move it to i915 core. Start
renaming stuff with the slightly more natural "initial plane" rather
than "plane initial".
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/cdad733192690a61fbb44921c57fc68cc1cd809f.1765812266.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>