MIPS: mm: Un-inline get_new_mmu_context

In preparation for adding MMID support to get_new_mmu_context() which
will increase the size of the function somewhat, move it from
asm/mmu_context.h into a C file.

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