drm/i915: Allow HDMI+VGA cloning

HDMI+VGA cloning should be supported on all platforms. The only real
obstacle is the 1.5x clock adjustment for 12bpc HDMI, but that is now
taken care of, so we can allow HDMI+VGA cloning.

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>