iommu/vt-d: Removed unused iommu_count in dmar domain

iommu/vt-d: Removed unused iommu_count in dmar domain

DMAR domain uses per DMAR refcount. It is indexed by iommu seq_id.
Older iommu_count is only incremented and decremented but no decisions
are taken based on this refcount. This is not of much use.

Hence, remove iommu_count and further simplify domain_detach_iommu()
by returning void.

Signed-off-by: Parav Pandit <parav@nvidia.com>
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Link: https://lore.kernel.org/r/20210530075053.264218-1-parav@nvidia.com
Link: https://lore.kernel.org/r/20210610020115.1637656-21-baolu.lu@linux.intel.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>