drm/i915: handle G45/GM45 pulse detection connected state.

In the HPD pulse handler we check for long pulses if the port is actually
connected, however we do that for IBX, but we use the pulse handling code on
GM45 systems as well, so we need to use a diffent check.

This patch refactors the digital port connected check out of the g4x detection
path and reuses it in the hpd pulse path.

Fixes: http://lkml.kernel.org/r/1409382202.5141.36.camel@marge.simpson.net
Reported-by: Mike Galbraith <umgwanakikbuti@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>