drm/radeon/dpm: simplify state adjust logic for NI

This is based on a similar patch from Alexandre Demers.
While fixing up some warnings with that patch I saw some
additional cleanups that could be applied.  This patch
simplifies the logic for patching the power state.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: Alexandre Demers <alexandre.f.demers@gmail.com>