drm/i915/chv: Use RMW to toggle swing calc init

The spec only tells us to set individual bits here and there. So we use
RMW for most things. Do the same for the swing calc init.

Eventually we should optimize things to just blast the final value in
with group access whenever possible. But to do that someone needs to
take a good look at what's the reset value for each registers, and
possibly if the BIOS manages to frob with some of them. For now
use RMW access always.

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>