drm/i915/display: remove ipc_enabled from struct drm_i915_private

The ipc_enabled member was supposed to be moved under the display wm
sub-struct, but due to a rebase fail only the new one was added and the
old one was left behind. Finish the job.

Fixes: 70296670 ("drm/i915/display: move IPC under display wm sub-struct")
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220916113850.3712354-1-jani.nikula@intel.com