Reduce use of GrPixelConfig inside of GrVkCaps.

This change removes all internal uses of GrPixelConfig in GrVkCaps, besides
translation of GrPixelConfig in overriden methods, as well as for swizzles
which actually live in shader caps.

Bug: skia:6718
Change-Id: I7a8bf03c9e6a7ed75a74862cb4c99cf610976fe8
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/213641
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
3 files changed