drm/radeon: check firmware overrides for mclk/sclk ss

Check the overrides in the firmware info table before
enabling spread spectrum on the engine or memory clocks.

Some boards may have valid spread spectrum tables, but
shouldn't necessarily have it enabled.

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

diff --cc drivers/gpu/drm/radeon/radeon_atombios.c
Simple merge