drm/i915: Extract {i9xx,ilk}_configure_cpu_transcoder()

Follow the path laid out by hsw+ and extract helpers to configure
the cpu transcoder for earlier platforms as well.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220128103757.22461-10-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>