virgl: fix an issue transfer extend
Quoting the commit message
In virgl_buffer_transfer_extend, when no flush is needed, it tries to extend a previously queued transfer instead if it can find one. Comparing to virgl_resource_transfer_prepare, it fails to check if the resource is busy.
The existence of a previously queued transfer normally implies that the resource is not busy, maybe except for when the transfer is PIPE_TRANSFER_UNSYNCHRONIZED. Rather than burdening us with a lengthy comment, and potential concerns over breaking it as the transfer code evolves, this commit makes the valid_buffer_range check the only condition to take the fast path.
In real world, we hit the fast path almost only because of the valid_buffer_range check. In micro benchmarks, the condition should always be true, otherwise the benchmarks are not very representative of meaningful workloads. I think this fix is justified.
The recent change to PIPE_TRANSFER_MAP_DIRECTLY usage disables the fast path. This commit re-enables it as well.
What do you think?