[Metal] Use Shared rather than Managed storageMode for M1 dynamic buffers

Change-Id: I53cb4306a9715f75e7b15f8d368c7d0b4007b7a1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/458956
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
3 files changed