drm/i915: Remove platform specific *_dp_detect() functions

Their logic is exactly the same: check if the digital port is connected
and then call intel_dp_detect_dpcd(). So just put that logic in their
only caller: intel_dp_detect().

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1447859970-9546-2-git-send-email-ander.conselvan.de.oliveira@intel.com