drm/amd/display: Only wait for flip pending on immediate flips

[Why]
We want to make sure all immediate flips are completed before locking
the pipes, but not pipes that are not flip immediate as they will be
locked by the OTG

[How]
Skip non flip immediate pipes when checking for flip pending.

Signed-off-by: Aric Cyr <aric.cyr@amd.com>
Acked-by: Bindu Ramamurthy <bindu.r@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>