drm/i915: use struct drm_device for clock gating funcs

drm/i915: use struct drm_device for clock gating funcs

While we want to refactor intel_clock_gating.[ch] and likely move a lot
of display related code to display, start off with a little intermediate
change to use struct drm_device in the interface instead of struct
drm_i915_private, to allow us to drop another dependency on i915_drv.h
and struct drm_i915_private.

Cc: Luca Coelho <luciano.coelho@intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Link: https://patch.msgid.link/20251121112200.3435099-2-jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>