drm/i915: Apply the force-detect VGA w/a to Valleyview

It appears that Valleyview shares its VGA encoder with more recent
siblings and requires the same forced detection cycle after a hardware
reset before we can rely on hotplugging.

Reported-and-tested-by: kobeqin <kobe.qin@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=67733
Tested-by: kobeqin <kobe.qin@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
[danvet: Check for gen >= 5 insted, acked by Chris.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>