drivers/gpu/drm/drm_ioc32.c: initialize all fields

The c32 structure is allocated on the stack and its idx field is not
initialized before copying it to user level.  This patch takes the value
from the result of the ioctl, as done for the other fields.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Dave Airlie <airlied@redhat.com>