drm/nouveau/fifo/gk104-: trigger mmu fault before attempting engine recovery

Greatly improves the chances of recovering the GPU from a CTXSW_TIMEOUT.

Tested with piglit's arb_shader_image_load_store-atomicity, which causes
GR to hang in such a way that recovery failed (CTXSW_TIMEOUT continually
re-triggers).

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