Fix issues with MTLPixelFormatBGR10A2Unorm on older OSes.

MTLPixelFormatBGR10A2Unorm isn't supported on older MacOS or iOS
versions, but we still want to build those. This CL adds some
availability checks, and works around array initialization by using a
internally defined constant.

Bug: skia:11160
Change-Id: Ife04b0a467a5e0aa27f081cabb1936c5771b5679
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/352742
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
2 files changed