drm/i915: Don't pass "mem_value" to ilk_compute_fbc_wm

The FBC watermark doesn't depend on the latency value, so no point in
passing it in.

Note: It actually depends upon the latency, but only through priv_val
...

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
[danvet: Add review comment from Paulo to the commit message.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>