nouveau: Make nouveau_wait_for_idle() read PTIMER.

Following my nv28 kmmio dumps, nouveau_wait_for_idle() is modified to
read PTIMER and NV03_PMC_ENABLE. Also a timeout based on PTIMER value is
added, so wait_for_idle() cannot stall indefinitely (unless PTIMER is
halted). The timeout was selected as 1 giga-ticks, which for me is 1s.