drm/i915: force full detect on sink count change

This patch checks for changes in sink count between short pulse
hpds and forces full detect when there is a change.

This will allow both detection of hotplug and unplug of panels
through dongles that give only short pulse for such events.

v2: changed variable type from u8 to bool (Jani)
    return immediately if perform_full_detect is set(Siva)

v3: changed method of determining full detection from using
    pointer to return code (Siva)

v4: changed comments to indicate meaning of return value of
    intel_dp_short_pulse and explain the use of return value
    from intel_dp_get_dpcd in intel_dp_short_pulse (Ander)

Tested-by: Nathan D Ciobanu <nathan.d.ciobanu@intel.com>
Signed-off-by: Sivakumar Thulasimani <sivakumar.thulasimani@intel.com>
Signed-off-by: Shubhangi Shrivastava <shubhangi.shrivastava@intel.com>
Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>
Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1459341326-13142-5-git-send-email-shubhangi.shrivastava@intel.com