drm/i915/gt: Hold request reference while waiting for w/a verification

As we wait upon a request, we must be holding a reference to it, and be
wary that i915_request_add() consumes the passed in reference.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191121093326.134774-1-chris@chris-wilson.co.uk