drm/amd/display: guard write a 0 post_divider value to HW

[why]
post_divider_value should not be 0.

Reviewed-by: Charlene Liu <charlene.liu@amd.com>
Signed-off-by: Ahmed, Muhammad <Ahmed.Ahmed@amd.com>
Signed-off-by: Zaeem Mohamed <zaeem.mohamed@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>