drm/i915: add DP support to intel_enable_ddi

We should only write the DDI_BUF_CTL at this point for HDMI/DVI. For
DP we need to do this earlier, and the values written to the register
are also different.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>