drm/amd/display: No audio endpoint for Dell MST display

[Why]
There are certain MST displays (i.e. Dell P2715Q)
that although have the MST feature set to off may still
report it is a branch device and a non-zero
value for downstream port present.
This can lead to us incorrectly classifying a
dp dongle connection as being active and
disabling the audio endpoint for the display.

[How]
Modified the placement and
condition used to assign
the is_branch_dev bit.

Signed-off-by: Harmanprit Tatla <harmanprit.tatla@amd.com>
Reviewed-by: Aric Cyr <aric.cyr@amd.com>
Acked-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>