drm/i915/dpio: s/pipe/ch/

Stop using 'pipe' directly as the DPIO PHY channel. This
does happen to work on VLV since it just has the one PHY
with CH0==pipe A and CH1==pipe B. But explicitly converting
the thing to the right enum makes the whole thing less
confusing.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240422083457.23815-9-ville.syrjala@linux.intel.com


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