drm/i915:gen9: restrict WaC6DisallowByGfxPause

WaC6DisallowByGfxPause is currently applied unconditionally
but is not required in all revisions.

v2: extend application of workaround to agree with w/a
database, which differs from the HSD.

References: HSD#2133391
Signed-off-by: Tim Gore <tim.gore@intel.com>
Reviewed-by: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1469008825-19442-1-git-send-email-tim.gore@intel.com