drm/i915/irq: initialize gen2_imr_mask in terms of enable_mask

drm/i915/irq: initialize gen2_imr_mask in terms of enable_mask

Instead of initializing gen2_imr_mask and enable_mask independently, use
the latter for initializing the former. This also highlights the
differences in the masks, i.e. what's set to enable_mask after it's been
used to initialize gen2_imr_mask.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://lore.kernel.org/r/e3b612ce4decea699bde2c52aeaef48bf95f7abc.1758637773.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>