powerpc: Make VSID_BITS* dependency explicit

VSID_BITS and VSID_BITS_1T depends on the context bits  and user esid
bits. Make the dependency explicit

Acked-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
CC: <stable@vger.kernel.org> [v3.8]