drm/nvc0/gr: move to exec engine interfaces

Much nicer to do that nv50, the code was pretty much written to expect
such a change in the future.

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

diff --cc drivers/gpu/drm/nouveau/nouveau_drv.h
Simple merge

diff --cc drivers/gpu/drm/nouveau/nouveau_state.c
Simple merge

diff --cc drivers/gpu/drm/nouveau/nvc0_graph.c
Simple merge

diff --cc drivers/gpu/drm/nouveau/nvc0_graph.h
Simple merge

diff --cc drivers/gpu/drm/nouveau/nvc0_grctx.c
Simple merge