drm/amdgpu: check for allocation failure in amdgpu_vkms_sw_init()

Check whether the kcalloc() fails and return -ENOMEM if it does.

Fixes: 84ec374b ("drm/amdgpu: create amdgpu_vkms (v4)")
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>