[graphite] Reset fDstCopy in resetCommandBuffer()

Bug: b/372413904
Bug: b/372298916
Bug: b/372404829
Bug: b/372404910
Change-Id: I78cfd9497091db04d41973314371e2ffb2087d57
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/909016
Commit-Queue: Nicolette Prevost <nicolettep@google.com>
Auto-Submit: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Nicolette Prevost <nicolettep@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>