drm/amdgpu: Add mask for SDMA instances

Add a mask of SDMA instances available for use. On certain ASIC configs,
not all SDMA instances are available for software use.

v2:
	Change sdma mask type to uint32_t (Le)

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Le Ma <Le.Ma@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>