drm/amd/display: Guard DST_Y_PREFETCH register overflow in DCN21

[why]
DST_Y_PREFETCH can overflow when DestinationLinesForPrefetch values are
too large due to the former being limited to 8 bits.

[how]
Set the maximum value of DestinationLinesForPrefetch to be 255 * refclk
period.

Reviewed-by: Laktyushkin Dmytro <dmytro.laktyushkin@amd.com>
Acked-by: Solomon Chiu <solomon.chiu@amd.com>
Signed-off-by: Victor Lu <victorchengchi.lu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>