drm/i915/power: move enum skl_power_gate under display

When the display registers were split off from i915_reg.h, enum
skl_power_gate was left behind. Move it to intel_display_regs.h.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://lore.kernel.org/r/495054983b74163ca7dcbf5a1b6a24099047bc64.1750855148.git.jani.nikula@intel.com


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