reland: quantize: simplify 32x32_b args

reland: quantize: simplify 32x32_b args

Allocate mb_plane_ on the heap to ensure src is aligned.

Now that all the implementations of the 32x32 quantize are in
intrinsics we can reference struct members directly. Saves
pushing them to the stack.

n_coeffs is not used at all for this function.

Change-Id: Ib551f7f583977602504d962b72063bc6eda9dda9