drm/i915: make a few DDI functions static

We don't need to export them since they're not being used outside the
file. The next time I try to find the callers for these things I will
know I won't need to look outside intel_ddi.c.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1490907472-10883-1-git-send-email-paulo.r.zanoni@intel.com