drm/i915/skl: Don't try to read out the PCH transcoder state if not present

When the platform doesn't have a FDI link, don't try to read out the
state of a potential PCH transcoder.

Reviewed-by: Thomas Wood <thomas.wood@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
[danvet: Don't open-code HAS_FDI if there's only one place that needs
it. Acked by Damien on irc.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>