drm/radeon/kms: spread spectrum fixes

- properly mask the ss type
- don't enable ss if type is external or percentage is 0
- if ss enabled and type is external, set ref_div_src to ext clock
- prefer ASIC_INTERNAL_SS_ON_DP to LCD_Info SS_Id for eDP
- fix ss amount calculation

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