Add table for supported colortypes in GrVkCaps.

Currently Vulkan does not have a second layer table for external IO
GrColorTypes since it doesn't need any extra meta data it for both
read and write it always uses the GrColorType of the surface.

Also removed a few switches in the GrVkCaps to use the new table.

Bug: skia:6718
Change-Id: Ib4df5862051a235df0c7c0acc0540a6b56dee516
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/232020
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
2 files changed