virgl: use debug_printf instead of fprintf

While we're at it, prefix the string with "VIRGL: ", to match similar
code elsewhere in virgl.

Fixes: d7b31969 ("virgl: Return an error if we use fp64 on top of GLES")
Signed-off-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Reviewed-by: Elie Tournier <elie.tournier@collabora.com>
15 jobs for !620 with virgl-debug_printf in 8 minutes and 49 seconds (queued for 2 seconds)
merge request