drm/nouveau: Fixup semaphores on pre-nv50 cards.

Apparently, they generate a PFIFO interrupt each time one of the
semaphore methods is executed if its ctxdma wasn't manually marked as
valid. This patch makes it flip the valid bit in response to the
DMA_SEMAPHORE method (which triggers the IRQ even for a valid ctxdma).

Signed-off-by: Francisco Jerez <currojerez@riseup.net>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>