drm/i915: don't leak dp_connector at intel_ddi_init

Regression introduced by:
    commit 311a2094
    Author: Paulo Zanoni <paulo.r.zanoni@intel.com>
        drm/i915: don't init DP or HDMI when not supported by DDI port

Since the commit above it is possible to have a DDI encoder that has
the HDMI connector but not the DP connector (in case the port doesn't
support DP). In this case, we must properly free the DP connector.

We just leak this once, so it's not a big deal.

Reported by kmemleak.

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