drm/amd/display: DCN2X Prefer ODM over bottom pipe to find second pipe

[WHY]
When finding a second pipe for pipe split, currently will look for
bottom pipe in context first to decide the second pipe. This causes
issues in 2 plane to 1 plane transitions like fullscreen video where
bottom pipe no longer exists in the new configuration.

[HOW]
If previous context had an ODM pipe, use that to find the secondary pipe
first before looking at bottom pipe.

Reviewed-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: sunglee <sunglee@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>