drm/i915/irq: use a dedicated IMR cache for VLV/CHV

drm/i915/irq: use a dedicated IMR cache for VLV/CHV

There are three groups of platforms using i915->irq_mask independently:
gen 2-4, VLV/CHV, and gen 5-7.

The VLV/CHV usage is purely limited to display. Move its irq_mask usage
to struct intel_display as vlv_imr_mask for VLV/CHV.

vlv_imr_mask could be put inside a union with de_irq_mask[], but keep
them separate to avoid accidental aliasing of the values.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://lore.kernel.org/r/cef6dee8d0b02ff76180c5879f3056e102947a57.1758198300.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>