drm/amd/display: Do not disable pipe split if mode is not supported

[WHY]
If mode is not supported, pipe split should not be disabled.
This may cause more modes to fail.

[HOW]
Check for mode support before disabling pipe split.

This commit was previously reverted as it was thought to
have problems, but those issues have been resolved.

Signed-off-by: Sung Lee <sung.lee@amd.com>
Reviewed-by: Yongqiang Sun <yongqiang.sun@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>