Simplify SkGpuBlurUtils::GaussianBlur method

No one was using the canClobberSrc capability and moving the direct filtering case forward makes the rest of the logic simpler.

Split out of: https://codereview.chromium.org/1959493002/ (Retract GrRenderTarget from SkGpuBlurUtils)

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Review-Url: https://codereview.chromium.org/1958603002