drm/amd/display: undo clearing of z10 related function pointers

[Why]
Z10 and S0i3 have some shared path. Previous code clean up ,
incorrectly removed these pointers, which breaks s0i3 restore

[How]
Do not clear the function pointers based on Z10 disable.

Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Pavle Kotarac <Pavle.Kotarac@amd.com>
Signed-off-by: Eric Yang <Eric.Yang2@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>