The initialization of "dmar_unit->gcmd" shall be done via reading from
Global Status Register rather than Global Command Register.
Rationale:
According to Chapter 10.4.4 Global Command Register in VT-d spec, Global Command
Register is a write-only register to control remapping hardware.
Global Status Register is the corresponding read-only register to report remapping
hardware status.
Tracked-On: #1842
Signed-off-by: Shiqing Gao <shiqing.gao@intel.com>