xf86drmMode: Unconditionally clear ioctl structs

We really have to do this to avoid surprises when extending the ABI
later on. Especially when growing the structures.

Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>