drm/i915: migrate hsw fdi code to new file.

Daniel asked for this, but it's a bit messy and I'm not sure
how best to clean it up yet.

Signed-off-by: Dave Airlie <airlied@redhat.com>
[Jani: also moved fdi buf trans to intel_fdi.c.]
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/44491f2465549ea5c2e48cde5437fa232f77ab96.1612467466.git.jani.nikula@intel.com