drm/i915: Check workaround status on dfs read time

As the workaround list has the value as initialization time
constant, we can do the simple checking on the go without
negleting igt.

Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Arun Siluvery <arun.siluvery@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>