[Mips] Remove isMoveReg=1 from wrdsp and rddsp instructions

This is a prep patch for D150388. Treating rddsp and wrdsp as copy
instructions was causing test failures as we tried using isCopyInstr()
hook to query target-specific copy instructions for LiveRangeSplitting.

As suggested, removing 'isMoveReg = 1' from wrdsp and rddsp so they
aren't considered simple copy-like instructions for the moment.

Reviewed By: sdardis

Differential Revision: https://reviews.llvm.org/D151181