msm: kgsl: Handle missed alloc error cases

There is a chance that we could fail to allocate memory and then write
to it when allocating entries for our scatter gather list.  Handle the
error cases properly to prevent writing to NULL.

Change-Id: I502fbb58817c10a7af8c3fb92e2602955f91accb
Signed-off-by: Carter Cooper <ccooper@codeaurora.org>