drm/i915: Merge i915_request.flags with i915_request.fence.flags

As we already have a flags field buried within i915_request, reuse it!

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200106114234.2529613-3-chris@chris-wilson.co.uk