drm/amdgpu: disable SDMA page queue on Vega20

Since we see driver loading failure on Vega20. Keep it
disabled until it's ready.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>