drm/i915/bxt: add support for HPD long/short pulse detection on HPD_PORT_A pin

This is a requirement for enabling display port HPD support on the port
A HPD pin. This support is to be added by follow-up patches.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Sonika Jindal <sonika.jindal@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>