drm/amd/display: fix NULL ptr deref in ISM delayed work

drm/amd/display: fix NULL ptr deref in ISM delayed work

dc_destroy() sets dm->dc to NULL before amdgpu_dm_ism_fini() is called,
leaving a window where in-flight ISM delayed work dereferences the stale
pointer. Call amdgpu_dm_ism_fini() in amdgpu_dm_fini() before dc_destroy().

Fixes: 754003486c3c ("drm/amd/display: Add Idle state manager(ISM)")
Reviewed-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Ray Wu <ray.wu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>