drm/amd/display: Prefer currently used OTG master when acquiring free pipe

drm/amd/display: Prefer currently used OTG master when acquiring free pipe

[WHY & HOW]
When acquiring an OTG master pipe we should prefer currently enabled OTG
master pipes first. If there are no free pipes used as current OTG
master pipe then we will try to acquire a currently unused free pipe as
new OTG master instead of tearing down current secondary pipes from ODM
or MPC combine.

Reviewed-by: Alvin Lee <alvin.lee2@amd.com>
Reviewed-by: Dillon Varone <dillon.varone@amd.com>
Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>