drm/amd/display: Uninitialized variables causing 4k60 UCLK to stay at DPM1 and not DPM0

[Why]
SwathSizePerSurfaceY[] and SwathSizePerSurfaceC[] values are uninitialized
 because we are using += instead of = operator.

[How]
Assign values in loop with = operator.

Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Samson Tam <samson.tam@amd.com>
Reviewed-by: Aric Cyr <aric.cyr@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>