drm/amd/display: Correct sw cache timing to ensure dispclk ramping

drm/amd/display: Correct sw cache timing to ensure dispclk ramping

[why]
Current driver will cache the dispclk right after send cmd to pmfw,
but actual clock not reached yet.

Change to only cache the dispclk setting after HW reached to the real clock.
Also give some range as it might be in bypass clock setting.

Reviewed-by: Yihan Zhu <yihan.zhu@amd.com>
Signed-off-by: Charlene Liu <Charlene.Liu@amd.com>
Signed-off-by: Ivan Lipski <ivan.lipski@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>