[Pixman] MIPS DSPr2: Fix for in_n_8 routine.

[Nemanja Lukic]

Increasing number of the iterations in blitters-test revealed bug in DSPr2
optimization. Bug is in the in_n_8 routine. Rounding logic was not implemented
right. Also, code used unnecessary multiplications, which could be avoided
by packing 4 destination (a8) pixel into one 32bit register. There were also,
unnecessary spills on stack. Code is rewritten to address mentioned issues.