virgl: Remove debugging left-overs

Some fprintfs were probably left unintentionally a few years ago and are
a bit of a nuisance.

Fixes: 2d3301e4 ("virgl: fix reference counting of prime handles")
       Cc: Rob Herring <robh@kernel.org>

Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
(cherry picked from commit 9b1cb50b)