drm/nouveau/fifo: remove all the "special" engine hooks

All the places this stuff is actually needed tends to be chipset-specific
anyway, so we're able to just inline the register bashing instead.

The parts of the common code that still directly touch PFIFO temporarily
have conditionals, these will be removed in subsequent commits that will
refactor the fifo modules into engine modules like graph/mpeg etc.

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