drm/i915: Store current watermark state in dev_priv->wm

To make it easier to check what watermark updates are actually
necessary, keep copies of the relevant bits that match the current
hardware state.

Also add DDB partitioning into hsw_wm_values as that's another piece
of state we want to track.

We don't read out the hardware state on init yet, so we can't really
start using this yet, but it will be used later.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
[danvet: Paulo asked for a comment around the memcmp to say that we
depend upon zero-initializing the entire structures due to padding.
But a later patch in this series removes the memcmp again. So this is
ok as-is.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>