drm/radeon: improve dac adjust heuristics for legacy pdac

commit 03ed8cf9b28d886c64c7e705c7bb1a365fd8fb95 upstream.

Hopefully avoid more quirks in the future due to bogus
vbios dac data.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>