Let's try to keep using the intermediate intel_pipe_wm representation
for as long as possible. It avoids subtle knowledge about the
internals of the hardware registers when trying to choose the
best watermark configuration.

While at it replace the memset() w/ zero initialization.
Signed-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: NPaulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>