winsys/svga: Resolve command submission buffer contention v3

winsys/svga: Resolve command submission buffer contention v3

If two contexts wanted to access the same buffer at the same time, it would
end up on two validation lists simultaneously, which might cause a
PIPE_ERROR_RETRY when trying to validate it from one context while the other
context already had it validated but not yet fenced.

In that situation we could spin until the error goes away, or apply various
more or less expensive locking schemes to save cpu.
Here we use a scheme that briefly locks after fencing but avoids locking on
validation in the non-contended case.

v2:
Make sure we broadcast not only on releasing buffers after fencing, but also
after releasing buffers in the pb_validate_validate error path.
v3:
Don't broadcast on PIPE_ERROR_RETRY because that would increase the chance
of starvation.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>