drm/amd/display: Disconnect DCN2 mpcc when changing tg

A previous fix was done for DCN1 that needed to be ported to DCN2:
commit 60c677534e73 ("drm/amd/display: Disconnect mpcc when changing tg")

Signed-off-by: Leo Li <sunpeng.li@amd.com>
Acked-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>