drm/i915: fix pipe DDI mode select

Mask the value before changing it and also select DVI when needed.

DVI was working in cases where the BIOS was setting the correct value
because we were not masking the value before changing it.

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