iommu/amd: Improve amd_iommu_v2_exit()

During module exit, the current logic loops through all possible
16-bit device ID space to search for existing devices and clean up
device state structures. This can be simplified by looping through
the device state list.

Also, refactor various clean up logic into free_device_state()
for better reusability.

Signed-off-by: Vasant Hegde <vasant.hegde@amd.com>
Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Link: https://lore.kernel.org/r/20220301085626.87680-6-vasant.hegde@amd.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>