drm/i915: Add some CHV DPIO lane power state asserts

Add some checks that the state of the DPIO lanes is more or less what we
expect based on the overrides.

The hardware only provides two bits per channel indicating whether all
or some of the lanes are powered down, so we can't do an exact check.

Additionally, CL2 powering down before we can check it adds another
twist. To work around this we simply check for the 0 value of the
CL2 register (which is what we get when it's powered down) and
adjust our expectations.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Deepak S <deepak.s@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>