drm/amd/display: reset the scaler boundary mode

[WHY]
The VBIOS select the black boundary mode when using auto
scale mode. But it doesn't recover if there is no reset.

[HOW]
Clean the scaler boundary mode to default edge in the manual
scale mode.

Reviewed-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Zhikai Zhai <zhikai.zhai@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>