drm/i915/gvt: properly check per_ctx bb valid state

Need to check valid state for per_ctx bb and bypass batch buffer
combine for scan if necessary. Otherwise adding invalid MI batch
buffer start cmd for per_ctx bb will cause scan failure, which is
taken as -EFAULT now so vGPU would be put in failsafe. This trys
to fix that by checking per_ctx bb valid state. Also remove old
invalid WARNING that indirect ctx bb shouldn't depend on valid
per_ctx bb.

Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Zhi Wang <zhi.a.wang@intel.com>