drm/amd/display: Fix NULL pointer dereference at hibernate

During hibernate sequence the source context might not have a clk_mgr.
So don't use it to look for DML2 support.

Link: https://gitlab.freedesktop.org/drm/amd/-/issues/2980


Fixes: 7966f319 ("drm/amd/display: Introduce DML2")
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>