VT-d: correct way to submit command to GCMD register

VT-d: correct way to submit command to GCMD register

Per VT-d spec, software should submit only one "incremental" command
at a time to Global Command reigster. Current implementation uses a
variable (gcmd) to record the state of Global Status register. It's
error prone.

Signed-off-by: Weidong Han <weidong.han@intel.com>