drm/amd/display: Don't apply FIFO resync W/A if rdivider = 0

[Description]
It is not valid to set the WDIVIDER value to 0, so do not
re-write to DISPCLK_WDIVIDER if the current value is 0
(i.e., it is at it's initial value and we have not made any
requests to change DISPCLK yet).

Reviewed-by: Saaem Rizvi <syedsaaem.rizvi@amd.com>
Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alvin Lee <alvin.lee2@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>