drm/i915: Split g4x_compute_pipe_wm() into two

Split g4x_compute_pipe_wm() into two halves. The first half computes
the new raw watermarks, and the second half munges those up into real
watermarks for the particular pipe.

We can reuse the second half for watermark sanitation as well.

Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220622155452.32587-2-ville.syrjala@linux.intel.com