MIPS: mm: Remove redundant preempt_disable in local_flush_tlb_mm()

The r4k variant of local_flush_tlb_mm() wraps its call to
drop_mmu_context() with a preempt_disable() & preempt_enable() pair, but
this is redundant since drop_mmu_context() disables interrupts and from
Documentation/preempt-locking.txt:

  Note that you do not need to explicitly prevent preemption if you are
  holding any locks or interrupts are disabled, since preemption is
  implicitly disabled in those cases.

Remove the redundant preempt_disable() & preempt_enable() calls.

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