Remove a few unneeded retain & autoreleases from Metal

We should avoid convenience factory methods when possible, to
keep things out of the autorelease pool.

Change-Id: I0a042b59619aff7a91b889add4ad341ab892809a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/381860
Auto-Submit: Adlai Holler <adlai@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
3 files changed