drm/i915: Introduce ilk_pch_disable() and ilk_pch_post_disable()

Hoover the remaining open coded PCH modeset sequence bits
out from ilk_crtc_disable(). Somewhat annoyingly the
enable vs. disable is a bit asymmetric so we need two
functions for the disable case.

Cc: Dave Airlie <airlied@redhat.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211015071625.593-8-ville.syrjala@linux.intel.com


Reviewed-by: Dave Airlie <airlied@redhat.com>