virgl: avoid large inline transfers

virgl: avoid large inline transfers

We flush everytime the command buffer (16 kB) is full, which is
quite costly.

This improves

dEQP-GLES3.performance.buffer.data_upload.function_call.buffer_data.new_buffer.usage_stream_draw

from 111.16 MB/s to 1930.36 MB/s.

In addition, I made the benchmark produce buffers from 0 --> VIRGL_MAX_CMDBUF_DWORDS * 4,
and tried ((VIRGL_MAX_CMDBUF_DWORDS * 4) / 2), ((VIRGL_MAX_CMDBUF_DWORDS * 4) / 4), etc.

I didn't notice any clear differences, so let's just go with the most obvious
heuristic.

Tested-By: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>