MIPS: DSPr2: Fix bug in over_n_8888_8888_ca/over_n_8888_0565_ca routines

MIPS: DSPr2: Fix bug in over_n_8888_8888_ca/over_n_8888_0565_ca routines

In main loop (unrolled by factor 2), instead of negating multiplied
mask values by srca, values of srca was negated, and passed as alpha
argument for

    UN8x4_MUL_UN8x4_ADD_UN8x4 macro.

Instead of:

    ma = ~ma;
    UN8x4_MUL_UN8x4_ADD_UN8x4 (d, ma, s);

Code was doing this:

    ma = ~srca;
    UN8x4_MUL_UN8x4_ADD_UN8x4 (d, ma, s);

Key is in substituting registers s0/s1 (containing srca value), with
t0/t1 containing mask values multiplied by srca.  Register usage is
also improved (less registers are saved on stack, for
over_n_8888_8888_ca routine).

The bug was introduced in commit d2ee5631 and revealed by composite test.