drm/i915: Introduce lpt_pch_disable()

Let's add lpt_pch_disable() as the counterpart to
lpt_pch_enable().

Note that unlike the ilk+ code the fdi_link_train()
and fdi_disable() calls are still left directly in
intel_crt.c. If we wanted to move those we'd need to
add lpt_pch_pre_enable(). But the two fdi direct fdi
calls are pretry symmetric so it doesn't seem too bad
to just keep them as is.

v2: Make lpt_disable_pch_transcoder() static (lkp@intel.com)

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/20211015071625.593-10-ville.syrjala@linux.intel.com


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