drm/i915/display: convert i915_pipestat_enable_mask() to struct intel_display

Going forward, struct intel_display is the main display device data
pointer. Convert i915_pipestat_enable_mask() to struct intel_display,
allowing further conversions elsewhere.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/975b382c703cfb62f24643e40eac247b8e8bbea8.1739378096.git.jani.nikula@intel.com