drm/i915: fix broadwell EU computation

subslice_mask is an array indexed by slice, not subslice.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Fixes: 8cc76693 ("drm/i915: store all subslice masks")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=108712


Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181112123931.2815-1-lionel.g.landwerlin@intel.com