drm/amd/display: Reset flip_immediate to topmost plane

[Why]
When checking if we want to disable GSL or not,
we should reset flip_immediate to be the flip type
of the topmost plane before looping through the
other planes.

[How]
Set flip_immediate to be the flip type of the topmost
plane before looping through the other planes.

Signed-off-by: Alvin Lee <alvin.lee2@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>