drm/i915: add ValleyView specific CRT detect function

Might be able to merge this back in at some point, but we're seeing bugs
with ADPA based detection, so keep it separate for now with explicit
hotplug trigger usage.

v2: drop superfluous debug message
v3: comment forced detection, need to debug (Eugeni)

Reviewed-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>