drm/i915: fix a limit check in hsw_compute_wm_results()

The '!' here was not intended.  Since '!' has higher precedence than
compare, it means the check is never true.

This regression was introduced in

commit 71fff20f
Author: Ville Syrjälä <ville.syrjala@linux.intel.com>
Date:   Tue Aug 6 22:24:03 2013 +0300

    drm/i915: Kill fbc_enable from hsw_lp_wm_results

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>