drm/i915/dp: Read the HPD status before trying to read the DPCD

Just like:

  Author: Damien Lespiau <damien.lespiau@intel.com>
  Date:   Wed Dec 12 19:37:22 2012 +0000

      drm/i915/hdmi: Read the HPD status before trying to read the EDID

But this time for DiplayPort.

v2: Adapt to the ibx_ name change and don't add commit hash (Chris
Wilson, Jani Nikula)

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>