drm/i915: Simplify up vlv watermark sanitation

We can simplify the vlv watermark sanitation by reusing the
second half of vlv_compute_pipe_wm() to convert the sanitized
raw watermarks into the proper form to be used as the
optimal/intermediate watermarks.

Also to be consistent with normal watermark computation the sanitized
watermarks should be all 0 for any disabled plane. Previously we
zeroed out the watermarks only up to the level (ie. PM2/5/DVDFS)
that was enabled.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220622155452.32587-5-ville.syrjala@linux.intel.com


Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>