Cleanup GrBicubicEffect

Cleanup GrBicubicEffect

- The base class already stores a color space xform, so just forward our
  factory argument and use that one.

- We had factories that took arbitrary coefficients, but they were only
  used in the unit test. Make the assumption of Mitchell's coefficients
  really hard-coded, which lets us skip transposing, storing, comparing,
  etc...

BUG=skia:

Change-Id: Ia1d4f72d32c201cba1da24d61b90250f383336d1
Reviewed-on: https://skia-review.googlesource.com/6394
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>