msm: kgsl: A few quick cleanups to improve stability

Fix a few possible pointer dereference possiblities in corner cases
and make the drawctxt cmdqueue head and tail pointers unsigned to
avoid any misunderstandings.

Change-Id: Ic0dedbadfb2c90b70903b6f5b00d232ca2c52728
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>