drm/i915: invert the log inside intel_prepare_ddi

Do an early return in case we don't have DDI instead of having the
whole function inside an "if" statement.

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