drm/amdgpu: set LDS_CONFIG=0x20 on Navy Flounder to fix a GPU hang (v2)

v2: squash in build fix

Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Marek Olšák <marek.olsak@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>