drm/nve0: allow specification of channel engine type in abi16 call

Previously, if either vram/gart handles were specified as ~0, the ioctl
call would fail.  In order to hack engine selection into the ioctl for
kepler, we now define (fb_ctxdma_handle == ~0) to mean "engine mask is
in tt_ctxdma_handle".

This approach also allows new userspace to detect lack of support for
non-PGRAPH channels on older kernels.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>