virgl: fix sync issue regarding discard/unsync transfers

virgl: fix sync issue regarding discard/unsync transfers

GL_MAP_INVALIDATE_BUFFER_BIT cannot be treated as
GL_MAP_INVALIDATE_RANGE_BIT naively.  When we run into

  ptr = glMapBufferRange(buf, 0, size,
          GL_WRITE_BIT|GL_MAP_INVALIDATE_BUFFER_BIT);
  memcpy(ptr, data1, size);
  glUnmapBuffer(buf);
  ptr = glMapBufferRange(buf, size, size,
          GL_WRITE_BIT|GL_MAP_UNSYNCHRONIZED_BIT);
  memcpy(ptr, data2, size);
  glUnmapBuffer(buf);

we never want data1 to be copy_transfer'ed.  Because that would mean
that data2 might overwrite valid data.

Signed-off-by: Chia-I Wu <olvaffe@gmail.com>
Reviewed-by: Alexandros Frantzis alexandros.frantzis@collabora.com
Fixes: a22c5df0794 ("virgl: Use buffer copy transfers to avoid waiting when mapping")
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>