drm/i915/chv: Force clock buffer enables

Try to force the PHY clock buffer enables to make the clock routing
work.

v2: Fix the pipe B case to actually enable CH0 clock buffers

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>