intel-iommu: fix the way in which handle_gcmd_write() handle writes to GCMD_REG

When handling writes to GCMD_REG, we should compare the new val with GSTS_REG
instead of the old val of GCMD_REG. The handle of VTD_GCMD_SRTP is a special
case. Some cases are not implemented yet.

Signed-off-by: Le Tan <tamlokveer@gmail.com>