drm/amd/powerplay: fix uninitialized variable

refresh_rate was not initialized when program
display gap.
this patch can fix vce ring test failed
when do S3 on Polaris10.

bug: https://bugs.freedesktop.org/show_bug.cgi?id=103102
bug: https://bugzilla.kernel.org/show_bug.cgi?id=196615


Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org