drm/i915: add I915_PARAM_HAS_VEBOX to i915_getparam

This will let userland only try to use the new ring
when the appropriate kernel is present

Signed-off-by: Xiang, Haihao <haihao.xiang@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>