drm/i915: Move intel_ddi_fdi_post_disable() to fdi code

drm/i915: Move intel_ddi_fdi_post_disable() to fdi code

Reanme intel_ddi_fdi_post_disable() to hsw_fdi_disable() and
relocate it next to all the other code dealing with FDI_RX.
intel_ddi.c has now been cleansed of FDI_RX.

In order to avoid exposing intel_disable_ddi_buf() outside
intel_ddi.c we can just open code the DDI_BUF_CTL write. The
enable side already has all that stuff open coded so
this actually is more symmetric. But we do need to remeber
to bring the intel_wait_ddi_buf_idle() call over from
inside intel_disable_ddi_buf().

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/20211015071625.593-9-ville.syrjala@linux.intel.com
Reviewed-by: Dave Airlie <airlied@redhat.com>