[magma] Always cpu map with write access

Fixes an access violation exception.

Change-Id: Iacb8122365f1702add0b02d26f4faf9e56f24500
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/github.com/intel/media-driver/+/732140
Reviewed-by: John Bauman <jbauman@google.com>
1 file changed