drm/i915/display: remove the display platform enum as unnecessary

drm/i915/display: remove the display platform enum as unnecessary

The display platform enums are not really needed for anything. Remove.

Without the enum, PLATFORM_UNINITIALIZED is also no longer needed for
keeping the first enum 0. Also need to switch from sp->subplatform to
sp->pciidlist as the check for array end.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/d20966f1d7a69a1e66768110b427be2fc611bcd2.1730144869.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>