drm/amd/display: Check other planes for iflip only if GSL already enabled

[Why]
We don't want GSL to be enabled when only updating
plane address

[How]
Only check other pipes for immediate flip if GSL is
already enabled

Signed-off-by: Alvin Lee <alvin.lee2@amd.com>
Acked-by: Bindu Ramamurthy <bindu.r@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>