drm/i915: fix Haswell FDI link disable path

This covers the "Disable FDI" section from the CRT mode set sequence.
This disables the FDI receiver and also the FDI pll.

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