drm/i915: Simplify up g4x watermark sanitation

We can simplify the g4x watermark sanitation by reusing the
second half of g4x_compute_pipe_wm() to convert the sanitized
raw watermarks into the proper form to be used as the
optimal/intermediate watermarks.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220622155452.32587-4-ville.syrjala@linux.intel.com


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