drm/amd/display: Init clock value by current vbios CLKs

[Why]
While booting into OS, driver updates DPP/DISP CLKs.
But init clock value is zero which is invalid.

[How]
Get current clocks value to update init clocks.
To avoid underflow.

Signed-off-by: Brandon Syu <Brandon.Syu@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>