drm/i915: Move some computations out from hsw_compute_wm_parameters()

Move the watermark max computations into haswell_update_wm(). This
allows keeping the 1/2 vs. 5/6 split code in one place, and avoid having
to pass around so many things. We also save a bit of stack space by only
requiring one copy of struct hsw_wm_maximums.

Also move the intel_wm_config out from hsw_compute_wm_parameters() and
pass it it. We'll have some need for it in haswell_update_wm() later.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>