drm/i915/overlay: Move i915 specific code into i915_overlay.c

drm/i915/overlay: Move i915 specific code into i915_overlay.c

Relocate the i915 driver specific parts of the overlay code
into i915_overlay.c. This leaves intel_overlay.c with just
the display specific code.

The one annoyance here is the DSPCLK_GATE_D register access
being done from i830_overlay_clock_gating(). The register
definition lives on the display side as we do need to access
it on other platforms there. Since it's just one register
and bit, I decided to just duplicate that part in i915_reg.h.

v2: Use kzalloc_obj()

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/20260226133855.17690-1-ville.syrjala@linux.intel.com