drm/i915/gvt: set ring buffer size to default for guc submission

When not using GuC submission, the ring buffer size for GVT context is
512KB which is the max size. When switching to GuC submission, the ring
buffer size is required to be less than 16KB. So use the GVT context
default ring buffer size if GuC submission is enabled.

Signed-off-by: Chuanxiao Dong <chuanxiao.dong@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170216063639.GA17107@intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>