drm/i915: set the correct function pointers for Haswell DP

This is the final remaining piece of Haswell DP enablement. After this
patch, just calling intel_dp_init on any port will make DP work. We
still do not do this because we're currently initializing HDMI on all
the ports, so if we replace intel_hdmi_init with intel_dp_init, we
will break HDMI, and we can't call both because they share the same
registers.

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>