iommu/amd: Reduce amount of MMIO when submitting commands

As newer, higher speed devices are developed, perf data shows that the
amount of MMIO that is performed when submitting commands to the IOMMU
causes performance issues. Currently, the command submission path reads
the command buffer head and tail pointers and then writes the tail
pointer once the command is ready.

The tail pointer is only ever updated by the driver so it can be tracked
by the driver without having to read it from the hardware.

The head pointer is updated by the hardware, but can be read
opportunistically. Reading the head pointer only when it appears that
there might not be room in the command buffer and then re-checking the
available space reduces the number of times the head pointer has to be
read.

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>