drm/i915: don't assert disabled FDI before disabling the FDI

On Haswell/LPT we must disable the PCH transcoder before we disable
the FDI, so don't check for disabled FDI there.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>