iommu/amd: Reduce delay waiting for command buffer space

iommu/amd: Reduce delay waiting for command buffer space

Currently if there is no room to add a command to the command buffer, the
driver performs a "completion wait" which only returns when all commands
on the queue have been processed. There is no need to wait for the entire
command queue to be executed before adding the next command.

Update the driver to perform the same udelay() loop that the "completion
wait" performs, but instead re-read the head pointer to determine if
sufficient space is available.  The very first time it is found that there
is no space available, the udelay() will be skipped to immediately perform
the opportunistic read of the head pointer. If it is still found that
there is not sufficient space, then the udelay() will be performed.

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