MIPS: mm: Remove redundant drop_mmu_context() cpu argument

The drop_mmu_context() function accepts a cpu argument, but it
implicitly expects that this is always equal to smp_processor_id() by
allocating & configuring an ASID on the local CPU when the mm is active
on the CPU indicated by the cpu argument.

All callers do provide the value of smp_processor_id() to the cpu
argument.

Remove the redundant argument and have drop_mmu_context() call
smp_processor_id() itself, making it clearer that the cpu variable
always represents the local CPU.

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