drm/amd/display: write segment pointer with mot enabled for mst

drm/amd/display: write segment pointer with mot enabled for mst

[Why]
Some mst branches NAK's segment pointer writes with mot disabled.
So reset of segment pointer to 0 should be performed with mot enabled.

[How]
Write segment pointer of mst branch devices with mot enabled.

Reviewed-by: Cruise Hung <cruise.hung@amd.com>
Signed-off-by: Meenakshikumar Somasundaram <meenakshikumar.somasundaram@amd.com>
Signed-off-by: Tom Chung <chiahsuan.chung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>