drm/i915/irq: split out i965_display_irq_postinstall()

Split out i965_display_irq_postinstall() similar to other platforms.

Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Link: https://lore.kernel.org/r/5d404dcd0c606d1cb11f2e09c45e151a75b5b2c6.1746536745.git.jani.nikula@intel.com


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