drm/radeon: additional gcc fixes for radeon_atombios.c

Newer versions of gcc seem to wander off into the weeds
when dealing with variable sizes arrays in structs.
Rather than indexing the arrays, use pointer arithmetic.

Fix up spread spectrum tables.

See bugs:
https://bugs.freedesktop.org/show_bug.cgi?id=66932
https://bugs.freedesktop.org/show_bug.cgi?id=66972
https://bugs.freedesktop.org/show_bug.cgi?id=66945



Signed-off-by: Alex Deucher <alexander.deucher@amd.com>