drm/i915: completely reset the value of DDI_FUNC_CTL

Don't rely on previous values already set on the register. Everything
we're not explicitly setting should be zero for now.

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>