drm/msm: Fix potential buffer overflow issue

In function submit_create, if nr_cmds or nr_bos is assigned with
negative value, the allocated buffer may be small than intended.
Using this buffer will lead to buffer overflow issue.

Signed-off-by: Kasin Li <donglil@codeaurora.org>
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>