radv: Add driconf to always drain waves before writing timestamps

radv: Add driconf to always drain waves before writing timestamps

UE4's Vulkan backend uses vkCmdWriteTimestamp with TOP_OF_PIPE
to measure how long a workload took in the GPU Benchmark. This is wrong
and writes the timestamp before the workload is actually finished,
making it seem like the GPU is much faster than it actually is.
This caused subsequent benchmark passes to contain way too big workloads,
which caused soft hangs on slower GPUs.

Fixes GPU hangs with Splitgate during automatic settings configuration.

Cc: mesa-stable
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22823>