msm: kgsl: Consolidate sysfs store functions

Many of our sysfs _store hooks grok unsigned ints.  Use a common
function to replace the various sscanf() and strtoul() calls
scattered throughout the code.

Change-Id: Ic0dedbad792a300f97ba36882a4346d2dc28e521
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>