drm/i915: Allow HDMI+HDMI cloning on g4x

BSpec is a bit unclear whether HDMI+HDMI cloning should work on g4x.
Tests on real hardware say that it does. Since g4x can't send
infoframes to more than one HDMI port anyway, we don't lose anything
by allow it.

For PCH platforms BSpec explicitly forbids HDMI+HDMI cloning.

Whether HDMI+HDMI cloning might also work on VLV is a bit unclear, but
since we'd at least lose the capability of sending infoframes to more
than one cloned HDMI port, it doesn't seem like a good idea to allow it.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=73850
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>