msm: kgsl: Use proper permissions for read-only files

If a sysfs file is read-only and does not have a store
function, the file permissions should not include writing.
This fixes a kernel warning when creating the file.

Change-Id: I0afd2033fce68e1e1d3df96f6d73ae27d01bea3f
Signed-off-by: Kyle Piefer <kpiefer@codeaurora.org>