drm/nvc0/copy: check PUNITS to determine which copy engines are disabled

On some Fermi chipsets (NVCE particularly) PCOPY1 doesn't exist.  And if
what I've seen on Kepler is true of Fermi too, chipsets of the same type
can have different PCOPY units available.

This should fix a v3.5 regression reported by a number of people effecting
suspend/resume on NVC8/NVCE chipsets.

Cc: stable@vger.kernel.org [3.5]
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>