msm: kgsl: Properly check error codes when allocating ringbuffer space

Failure to allocate space can return error codes as well as NULL. Check
for both of these conditions when trying to allocate space for commands
in the ringbuffer.

Change-Id: Id1b025a2478ab9ee2c22959aeb2a5d83f54d943a
Signed-off-by: Carter Cooper <ccooper@codeaurora.org>