drm/amd/display: dmcu wait loop calculation is incorrect in RV

[Why]
Driver already get display clock from SMU base on MHz, but driver read
again and mutiple 1000 cause wait loop value is overflow.

[How]
remove coding error

Signed-off-by: Paul Hsieh <paul.hsieh@amd.com>
Reviewed-by: Eric Yang <eric.yang2@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>