drm/amd/display: Extract temp drm mst deallocation wa into its own function

[why]
A recent change was made to implement temporary workaround due DRM
update in MST interfaces. The workaround is added into our generic
deallocation MST sequence.

This ticket is to extract this temporary workaround into its own function so
it is differentiated from our generic sequence.

Reviewed-by: Jerry Zuo <Jerry.Zuo@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>