drm/i915/irq: use a dedicated IMR cache for gen 5-7

drm/i915/irq: use a dedicated IMR cache for gen 5-7

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

The gen 5-7 usage is primarily limited to display. Move its irq_mask
usage to struct intel_display as ilk_de_imr_mask for gen 5-7.

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

With this, we can also drop the irq_mask member from struct xe_device.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://lore.kernel.org/r/adf60e74b890d52dd20ab4673111ae2063d33b49.1758198300.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>