drm/i915: Introduce ilk_pch_pre_enable()

Complete the ilk pch modeset abstraction by adding ilk_pch_pre_enable().
I did the disable vs. post_disable split already for the disable
sequence, but the enable sequence was still left with the naked
ilk_fdi_pll_enable() call for some reason.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220124193136.2397-2-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>