MIPS: tlbex: Simplify r3k check

We already know whether a CPU has r3k style exceptions, including TLB
exceptions, by checking cpu_has_3kex. Remove the list of CPU types in
build_tlb_refill_handler() & check cpu_has_3kex instead.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: linux-mips@vger.kernel.org