[Pixman] More MIPS DSPr2 bilinear fast paths.

[Nemanja Lukic]

Added optimizations for several bilinear fast paths:
 - src_8888_8_8888
 - src_8888_8_0565
 - src_0565_8_x888
 - src_0565_8_0565
 - add_8888_8_8888
 - src_8888_8888
 - src_8888_0565
 - src_0565_8888
 - src_0565_0565
 - over_8888_8888
 - add_8888_8888
Benchmark results (using lowlevel-blt-bench) on Malta board (@1Ghz) are included in the log messages.
Per previous code review:
 - Assembly code now respects the interpolation precision settings defined by BILINEAR_INTERPOLATION_BITS.
 - "make check" pass when this flag is set to 7 (default) and 8.
Any comments to this patch are welcome.