drm/i915: Compute vlv/chv wms the atomic way

Start computing the vlv/chv watermarks the atomic way, from the
.compute_pipe_wm() hook. We'll recompute the actual watermarks
for only planes that are part of the state, the other planes will
keep their watermark from the last time it was computed.

And the actual watermark programming will happen from the
.initial_watermarks() hook. For now we'll just compute the
optimal watermarks, and we'll hook up the intermediate
watermarks properly later.

The DSPARB registers responsible for the FIFO paritioning are
double buffered, so they will be programming from
intel_begin_crtc_commit().

v2: s/noninverted/raw/ for consistency with other platforms
    s/vlv_plane_wm_set/vlv_raw_plane_wm_set/ for clarity

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170302171508.1666-8-ville.syrjala@linux.intel.com