drm/radeon: fix a couple of array index errors

There are a couple of array overruns, and some associated confusion in
the code.
This is just a wild guess at what the code should actually look like.

Coverity CID: 13305 13306

agd5f: fix up the original intent of the timing code

Signed-off-by: Darren Jenkins <darrenrjenkins@gmail.com>
Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>