drm/amdgpu: Use kvmalloc for CS chunks

The number of chunks/chunks_array may be passed in
by userspace and can be large.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Chen Li <chenli@uniontech.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>