drm/i915/dsi: convert platform checks to display->platform.<platform> style

These are stragglers from a time the display->platform mechanism didn't
exist. Finish the conversion.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/493e4c550f9c515e2e82df1afd8a74a24156e76e.1739378096.git.jani.nikula@intel.com