drm/i915/cx0: convert to struct intel_display based platform checks

Switch the IS_<PLATFORM>() checks to display->platform.<platform>, and
drop a number of struct drm_i915_private pointers in the process.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250204134228.2934744-3-jani.nikula@intel.com


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