[magma] Only log unimplemented once

Don't spam the log with unimplemented log messages. Instead log the
message when the function is first called and then don't log again.

Change-Id: I849063854690b8f97fc749db73681192f7721806
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/github.com/intel/media-driver/+/746643
Reviewed-by: Craig Stout <cstout@google.com>