Although the clamping for the inverse transforms is still being
discussed, it is useful to have clear code.

The current code is not ideal - both av1_inv_txfm1d.c and av1_inv_txfm2d.c
have functions called clamp_buf(), but they do different things - one
clamps to the exact number of bits passed in, the other optionally
clamps to 16 bits.

This commit refactors things so that:

* clamp_* always clamps to the exact number of bits passed in
  To compensate, we push the "if bit <= 16: clamp to 16 bits"
  logic into the callers of the clamp_* functions

* We deduplicate some of the code

BUG=aomedia:1352

Change-Id: I1b9d85d6db5d291814c055a8113b7b054690b04c