drm/nv50-/disp: use the number of dac, sor, pior rather than hardcoded values

The values are already stored on chipset specific basis in the ctor.
Make the most of them and simplify the code further by using a temporary
variable to avoid code duplication.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>