drm/amdgpu: enable async gfx ring by default

VDDGFX requires gfx queue to be installed via MAP_QUEUES packet.
Hence, enable async gfx ring by default.

Signed-off-by: Jack Xiao <Jack.Xiao@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>