drm/amd/display: Check top sink only when multiple streams for DP2

[why]
When switching from extended to second display only
mode, the top remote sink is not removed while the top stream
itself is released. This causes DML to think there is no
DP2 output encoder because top remote sink does not match
with the second stream and disables DTBCLK and causes
hang.

[how]
For DP2.0 MST hubs, only treat 1st remote sink as an encoder
only when there are multiple displays connected.

Reviewed-by: Michael Strauss <michael.strauss@amd.com>
Signed-off-by: Sung Joon Kim <sungjoon.kim@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>