drm/amd/display: Populate invalid split index to be 0xF

[why]
There exists scenarios where the split index for subvp can be
pipe index 0. The assumption in FW is that the split index
won't be 0 but this is incorrect.

[how]
Instead populate non-split cases to be 0xF to differentiate
between split and non-split.

Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Alvin Lee <alvin.lee2@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>