drm/i915/gvt: drop checks for early Skylake revisions

We no longer cater for pre-production revisions of Skylake.

Fixes: d4362225 ("drm/i915/gvt: update misc ctl regs base on stepping info")
Cc: Ping Gao <ping.a.gao@intel.com>
Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Cc: Zhi Wang <zhi.a.wang@intel.com>
Cc: <intel-gfx@lists.freedesktop.org>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>