radv: Dump command buffer on hang.

radv: Dump command buffer on hang.

v2:
  - Now use the filename specified by RADV_TRACE_FILE env var.
  - Use the same var to enable tracing.

I thought we could as well always set the filename explicitly
instead of having some arbitrary defaults, and at that point
we don't need a separate feature enable.

Signed-off-by: Bas Nieuwenhuizen <basni@google.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>