drm/amdgpu/display: disable prefer_shadow for generic fb helpers

Seems to break hibernation.  Disable for now until we can root
cause it.

Fixes: 087451f3

 ("drm/amdgpu: use generic fb helpers instead of setting up AMD own's.")
Bug: https://bugzilla.kernel.org/show_bug.cgi?id=216119
Acked-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>