drm/amdgpu/powerplay: check vrefresh when when changing displays

Compare the current vrefresh in addition to the number of displays
when determining whether or not the smu needs updates when changing
modes. The SMU needs to be updated if the vbi timeout changes due
to a different refresh rate.  Fixes flickering around mode changes
in some cases on polaris parts.

Reviewed-by: Rex Zhu <Rex.Zhu@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>