msm: kgsl: Fix several possible problem spots in the dispatcher

Fix a few spots that the compiler warnings missed. Avoid
dereferencing a pointer that may have returned ERR_PTR() without
first checking to see that if it did return ERR_PTR() and correct
an egregious signed / unsigned mixup. Not sure why the compiler
didn't yell at me for that one.

Change-Id: Ic0dedbadf075b34e51a89ce625d2308c5171a58d
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>