msm: kgsl: Correctly limit max number of contexts per process

Read and increment context count atomic variable under a lock
to avoid race condition between read and increment. This is
necessary to make sure no process goes beyond the specified
context limit.

Change-Id: I483e2ac169beaff49e19b8ef1b46541f6eb740b0
Signed-off-by: Deepak Kumar <dkumar@codeaurora.org>