Don't pass in RT to individual Gpu CommandBuffer calls

The Vulkan backend already stored a GrVkRT, but was inconsistent
in sometimes using the stored value and sometimes the passed in
value (though they should be the same). This just cleans up the
code so that everyone uses a stored RT.

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=3126

Change-Id: I571de4bfb1da612d61171321d5224a9a19d8e545
Reviewed-on: https://skia-review.googlesource.com/3126
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
9 files changed