MIPS: dspr2: Added optimization for DCMode function

Change-Id: I8ea31907c1ea1259ec4db8cee1a479bd13a025a1
simplify the Histogram struct, to only store max_value and last_nz

we don't need to store the whole distribution in order to compute the alpha

Later, we can incorporate the max_value / last_non_zero bookkeeping
in SSE2 directly.

Change-Id: I748ccea4ac17965d7afcab91845ef01be3aa3e15
Merge "simplify the Histogram struct, to only store max_value and last_nz"