kR8_unorm_SkColorType: Add D3D and Metal support

This was missed in GrD3DCaps and GrMtlCaps.

Also fix SkPixmap::getColor4f, which had an extra factor of 255 in some
SkColorTypes.

Change-Id: Id07095f8250b2a6011b95fb5e94191bd324e285f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1154296
Commit-Queue: ccameron chromium <ccameron@chromium.org>
Reviewed-by: Nicolette Prevost <nicolettep@google.com>
Reviewed-by: Christopher Cameron <ccameron@google.com>
Commit-Queue: Christopher Cameron <ccameron@google.com>