filters_mips_dsp_r2.c: disable unfilters

the output does not match the C-code.

Change-Id: Ie7b7387478a6b5c3f08691628ae00f059cf6d899
Enable entropy based merge histo for (q<100)

Enable bin-partition entropy based heuristic for merging histograms
for higher (q >= 90) qualities as well. Keep the old behavior at the
maximum quality level (q==100).

This speeds up the compression between Q=90-99 (method=4) by factor 5-7X
and with loss of 0.5-0.8% in the compression density.

Change-Id: I011182cb8ae5403c565a150362bc302630b3f330