drm/amd/display: Disable pipe split for modes with borders

[Why]
For some special timing with border, like DMT 640*480 72Hz,
pipe split can't handle well. Thus, it will be black screen
for these special timing.

[How]
Disable pipe split for these timing with borders as W/A.

Signed-off-by: Dale Zhao <dale.zhao@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>