drm/i915: Add RPL-U sub platform

Separate out RPLU device ids and add them to both RPL and
newly created RPL-U subplatforms.

v2: (Matt)
    - Sort PCI-IDs numerically
    - Name the sub-platform to accurately depict what it is for
    - Make RPL-U part of RPL subplatform

v3: revert to RPL-U subplatform (Jani)

v4: (Jani)
    - Add RPL-U ids to RPL-P platform
    - Remove redundant comment

Signed-off-by: Chaitanya Kumar Borah <chaitanya.kumar.borah@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230130100806.1373883-2-chaitanya.kumar.borah@intel.com