drm/i915/irq: split out i915_display_irq_postinstall()

Split out i915_display_irq_postinstall() similar to other platforms.

Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Link: https://lore.kernel.org/r/11de06206ff10c27104b0ac3efda085bf4c1f1a6.1746536745.git.jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>