remove 8-bit Params struct

I'm not sure why I wrote this to use a Params struct originally, but we
should have plenty of registers in _8bit to pass everything directly and
avoid the stack.  Even once we enable the 8-bit pipeline on 32-bit x86,
we'll have 4 general purpose registers and 4 vector registers to use,
precisely what we're using here.

Change-Id: I3e51ab73186edcdcb8bfaa6cc99d9516db7c032a
Reviewed-on: https://skia-review.googlesource.com/40771
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
3 files changed