msm: kgsl: Access dma_buf_attachment pointer only if its valid

dma_buf_attach can return a negative error code wrapped
into a pointer on failure. Correct the error check and move
access to dma_buf_attachment pointer after the error check
to avoid accessing invalid pointer.

Change-Id: I77426fefb17370da5ed57878632bec8bacbc77a0
Signed-off-by: Deepak Kumar <dkumar@codeaurora.org>