drm/amdgpu: handle bo size 0 in amdgpu_bo_create_kernel_at (v2)

Just return early to match other bo_create functions.

v2: check if the bo_ptr is NULL rather than checking the size.

Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com> (v1)
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>