drm/i915/selftests: Drop local vm reference!

After obtaining a local reference to the vm from the context, remember
to drop it before it goes out of scope!

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191128185402.110678-1-chris@chris-wilson.co.uk