drm/nouveau/core: have fifo store a unique context identifier at attach time

This value will match something that's easily available from the engine IRQ
handlers, and used to lookup the relevant context.

Since the changes in how this is done on each generation match when the
major PFIFO changes happened, fifo is responsible for calculating the
correct value to avoid duplicating the same code among many engine modules.

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