drm/i915/display: add subplatform group for HSW/BDW ULT

drm/i915/display: add subplatform group for HSW/BDW ULT

Add support for defining aliases for subplatform groups, such as HSW/BDW
ULT that covers both ULT and ULX.

ULT is a special case, because we slightly abuse the ULT subplatform
both as a subplatform and group, but with the way this is defined, it
should be fairly clear.

This follows i915 core and IS_HASWELL_ULT()/IS_BROADWELL_ULT()
conventions, i.e. "is ULT" also matches ULX platforms.

Note: Pedantically, this should have been done earlier, but it's only
feasible now that we no longer have a subplatform enum and can actually
initialize multiple subplatforms.

v2: Use the subplatform group idea

Reviewed-by: Rodrigo Vivi <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/d148b6210a561b874642ae3e0ad10073d0615de7.1730144869.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <[email protected]>