drm/i915: Extract intel_attach_scaling_mode_property()

Consolidate the scaling_mode property setup into a single
place.

The one slight complicateion here is that GMCH platforms can't
do the CENTER mode except on the LVDS port. But we can deal with
that by just checking the connector type.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220912111814.17466-12-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>