Remove second version of SkImageInfo2GrPixelConfig

Alpha type is not (and never will be) part of pixel config, so the logic
around that was unnecessary. (Also, we already sanitize color type and
alpha type before making a new device at a higher level).

With that out of the way, we can easily supply a full info at the two
call-sites that were using the other version.

BUG=skia:

Change-Id: Iceccdbdebd1062d3e5023620755aabcc86604d2f
Reviewed-on: https://skia-review.googlesource.com/6920
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
4 files changed