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>

drivers/gpu/drm/nouveau/nouveau_chan.c

@@ -346,7 +346,7 @@ nouveau_channel_init(struct nouveau_channel *chan, u32 vram, u32 gart)
 	/* allocate software object class (used for fences on <= nv05, and
 	 * to signal flip completion), bind it to a subchannel.
 	 */
-	if (chan != chan->drm->cechan) {
+	if ((device->card_type < NV_E0) || gart /* nve0: want_nvsw */) {
 		ret = nouveau_object_new(nv_object(client), chan->handle,
 					 NvSw, nouveau_abi16_swclass(chan->drm),
 					 NULL, 0, &object);