drm/i915: split intel_dp_init into encoder and connector pieces

Same reason as the previous HDMI commit: the DDI code will have its
own encoder init function but still use the DP and HDMI connectors.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
[danvet: kill the unnecessarily added line that Damien spotted in
review.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>