drm/i915/gt: Use the BIT when checking the flags, not the index

drm/i915/gt: Use the BIT when checking the flags, not the index

In converting over to using set_bit()/test_bit(), when manually
inspecting the rq->fence.flags, we need to use BIT().

Fixes: 51aa0406a02e ("drm/i915: Merge i915_request.flags with i915_request.fence.flags")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200115122509.2673075-1-chris@chris-wilson.co.uk