drm/amd/display: fix up reference clock abstractions

[why]
"reference clock" is a very overloaded variable in DC and causes confusion
as there are multiple sources of reference clock, which may be different values
incorrect input values to DML will cause DCHUB to be programmed improperly
and lead to hard to debug underflow issues

[how]
instead of using ref clock everywhere, specify WHICH ref clock:
- xtalin
- dccg refclk
- dchub refclk

these are all distinct values which may not be equal

Signed-off-by: Jun Lei <Jun.Lei@amd.com>
Reviewed-by: Yongqiang Sun <yongqiang.sun@amd.com>
Acked-by: David Francis <David.Francis@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>