virgl: Fix flush in virgl_encoder_inline_write.

The current code is buggy: if there are only 12 dwords left in cbuf,
we emit a zero data length command which will be rejected by virglrenderer.
Fix it by calling flush in this case.

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Dave Airlie <airlied@redhat.com>
(cherry picked from commit 04e278f7)