drm/amd/display: Fix black flash when switching from ODM2to1 to ODMBypass

[Why]
On secondary display hotplug we switch primary
stream from ODM2to1 to ODMBypass mode. Current
logic will trigger disabling front end for this
stream.

[How]
We need to check if prev_odm_pipe is equal to NULL
in order to disable dangling planes in this scenario.

Reviewed-by: Ariel Bernstein <Eric.Bernstein@amd.com>
Acked-by: Brian Chang <Brian.Chang@amd.com>
Signed-off-by: Vladimir Stempen <vladimir.stempen@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>