drm/amd/display: fix multi display seamless boot case

[Why]
There is a scenario that causes eDP to become blank if
there are multiple displays connected, and the external
display is set as the primary display such that the first
flip comes to the external display.

In this scenario, we call our optimize function before
the eDP even has a chance to flip.

[How]
There is a check that prevents bandwidth optimize from
occurring before first flip is complete on the seamless boot
display.
But actually it assumed the seamless boot display is the
first one to flip. But in this scenario it is not.
Modify the check to ensure the steam with the seamless
boot flag set is the one that has completed the first flip.

Signed-off-by: Anthony Koo <anthony.koo@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>