drm/i915: s/INTEL_OUTPUT_DISPLAYPORT/INTEL_OUTPUT_DP/

INTEL_OUTPUT_DISPLAYPORT hsa been bugging me for a long time. It always
looks out of place besides INTEL_OUTPUT_EDP and INTEL_OUTPUT_DP_MST.
Let's just rename it to INTEL_OUTPUT_DP.

v2: Rebase

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1466621833-5054-9-git-send-email-ville.syrjala@linux.intel.com