msm: kgsl: Fix address generation for SVM buffers

Fix various corner cases and various mathmatical mistakes in the
SVM address path.  End result should be clearer and (hopefully)
more correct code.

Change-Id: Ic0dedbad1418609913b4cec7781680bbe0be78ff
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>