drm/xe/display: Add init_clock_gating.h stubs

drm/xe/display: Add init_clock_gating.h stubs

Add static inline stubs for init_clock_gating.h functions
so that we don't need ifdefs in the actual code. We already
have one in intel_display_power.c, and now I need to bring
over intel_display_reset.c.

Cc: Jouni Högander <jouni.hogander@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/20260415210411.24750-6-ville.syrjala@linux.intel.com