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>

src/gallium/drivers/virgl/virgl_resource.c

@@ -95,7 +95,11 @@ static void virgl_buffer_subdata(struct pipe_context *pipe,
       usage |= PIPE_TRANSFER_DISCARD_RANGE;
 
    u_box_1d(offset, size, &box);
-   virgl_transfer_inline_write(pipe, resource, 0, usage, &box, data, 0, 0);
+
+   if (size >= (VIRGL_MAX_CMDBUF_DWORDS * 4))
+      u_default_buffer_subdata(pipe, resource, usage, offset, size, data);
+   else
+      virgl_transfer_inline_write(pipe, resource, 0, usage, &box, data, 0, 0);
 }
 
 void virgl_init_context_resource_functions(struct pipe_context *ctx)