drm/i915/selftests: Move mock_vma to the heap to reduce stack_frame

An i915_vma struct on the stack may push the frame over the limit, if
set conservatively, so move it to the heap.

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/20191125124856.1761176-1-chris@chris-wilson.co.uk