drm/i915: Init HSW watermark tracking in intel_modeset_setup_hw_state()

Fill out the HSW watermark s/w tracking structures with the current
hardware state in intel_modeset_setup_hw_state(). This allows us to skip
the HW state readback during watermark programming and just use the values
we keep around in dev_priv->wm. Reduces the overhead of the watermark
programming quite a bit.

v2: s/init_wm/wm_get_hw_state
    Remove stale comment about sprites
    Make DDB partitioning readout safer

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
[danvet: Fix whitespace fail.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>