drm/amd/display: use dispclk AVFS for dppclk

[Why]
There is using pixelclk AVFS for dppclk, that would cause issue.

[How]
To use dispclk AVFS for both dispclk and dppclk.  There would choose
dppclk for request voltage when dispclk wouldn't be updated case.  If
dispclk need to be updated, then it'll choose the bigger one from dppclk
and dispclk for request voltage.

Signed-off-by: Brandon Syu <Brandon.Syu@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>