msm: kgsl: Avoid a possible NULL reference in kgsl_ioctl_helper

It is possible to dereference a NULL pointer through a series of
conditions in kgsl_ioctl_helper. Define the pointer at initialization
to ensure that the compiler does the right thing.

Change-Id: Ic0dedbada5f00bd41960c8bbdb4efe28903cbd4b
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>