drm/i915/gvt: Hold a reference on the request

The workload took a pointer to the request, and even waited upon,
without holding a reference on the request. Take that reference
explicitly and fix up the error path following request allocation that
missed flushing the request.

v2: [zhenyuw]
- drop request put in error path for dispatch, as main thread
caller will handle it identically to a real request.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>