drm/amd/display: Avoid programming DTO if Refclk is 0

[Why]
Reference clock, either DPREFCLK or DTBCLK can be a value of 0
which then will encounter division by 0.

[How]
Avoid further calculation and programming if refclk is not
populated.

Reviewed-by: Dillon Varone <dillon.varone@amd.com>
Acked-by: Zaeem Mohamed <zaeem.mohamed@amd.com>
Signed-off-by: Chris Park <chris.park@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>