virgl: fix a sync issue in virgl_buffer_transfer_extend

virgl: fix a sync issue in virgl_buffer_transfer_extend

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.

Signed-off-by: Chia-I Wu <olvaffe@gmail.com>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>