virgl: Remove wrong EAGAIN handling for drmIoctl

drmIoctl handles EAGAIN itself and actually it always return -1 on errors.
Remove the wrong handling of its return value. Also, print a warning when
it fails.

v2: - use _debug_printf instead of fprintf (Gurchetan Singh)

Signed-off-by: Lepton Wu <lepton@chromium.org>
Reviewed-by: Eric Anholt <eric@anholt.net> (v1)