drm/i915/overlay: Make i830_overlay_clock_gating() i915 specific

drm/i915/overlay: Make i830_overlay_clock_gating() i915 specific

i830_overlay_clock_gating() will remain on the i915 side of the
parent vs. display driver split. Stop using display specific stuff
inside it.

The one annoyance here is access to the display engine's
DSPCLK_GATE_D register. The proper way to deal with that might
be to move it to the display side, but that seems a bit hard right
now. So leave it where it is for now.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/20260226100738.29997-15-ville.syrjala@linux.intel.com