drm/i915/display: add support for subplatforms

drm/i915/display: add support for subplatforms

Add support for subplatforms. This is similar to what the xe driver is
doing. The subplatform is an enum and it's exclusive, i.e. only one
subplatform can match, and it completely identifies the platform and
subplatform. This is different from i915 core, and is notable in the
handling of ULT/ULX and RPL/RPL-U.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/8c04e32648395c0b745bc31a1edd4ef6f574bb70.1716399081.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>