drm/i915: set FDI translations to NULL on SKL

drivers/gpu/drm/i915/intel_ddi.c: In function ‘intel_prepare_ddi’:
drivers/gpu/drm/i915/intel_ddi.c:517:6: warning:
‘ddi_translations_fdi’ may be used uninitialized in this function
[-Wmaybe-uninitialized]
   if (ddi_translations_fdi)
      ^
drivers/gpu/drm/i915/intel_ddi.c:446:30: note: ‘ddi_translations_fdi’
was declared here
  const struct ddi_buf_trans *ddi_translations_fdi;
                              ^

This line used to be there, but was removed by:

commit f8896f5d
Author: David Weinehall <david.weinehall@linux.intel.com>
Date:   Thu Jun 25 11:11:03 2015 +030
    drm/i915/skl: Buffer translation improvements

Cc: David Weinehall <david.weinehall@linux.intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>