drm/i915/gvt: set the correct default value of CTX STATUS PTR

Fix wrong initial csb read pointer value. This fixes the random
engine timeout issue in guest when guest boots up.

Fixes: 8453d674

 ("drm/i915/gvt: vGPU execlist virtualization")
Cc: stable@vger.kernel.org # v4.10+
Signed-off-by: Min He <min.he@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>