drm/i915: Do not hardcode s_max, ss_max and eu_mask for BXT

We can calculate BXT values correctly from GFX fuse values without
hardcoding special limits.

Cc: Imre Deak <imre.deak@intel.com>
Cc: Matthew D Roper <matthew.d.roper@intel.com>
Signed-off-by: Dongwon Kim <dongwon.kim@intel.com>
Reviewed-by: Arun Siluvery <arun.siluvery@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>