drm/i915/irq: move all PCH irq postinstall calls to display code

Unify on making the calls from display code. Need to add an if ladder in
gen8_de_irq_postinstall() for now, but the function looks like it could
be overall be better split by platform. Something for the future.

The display version check for mtp seems a bit suspect, but this matches
current code.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/fe51744aec9e2f465caf0d699b8a15591859f89e.1691509966.git.jani.nikula@intel.com