drm/amdkfd: Fix NULL pointer dereference for set_scratch_backing_va()

Currently this function pointer is missing for GFX10. Considering it is
a void function since GFX9, fix it by checking the function pointer
before dereferencing it.

Signed-off-by: Yong Zhao <Yong.Zhao@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>