drm/amdgpu: Convert select_sched into a common helper v3

drm/amdgpu: Convert select_sched into a common helper v3

The xcp select_sched function does not need to
remain as a soc specific callback. It can be reused
for future products

v2: bypass the function if xcp_mgr is not available (Likun)

v3: Let caller check the availability of xcp mgr (Lijo)

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