msm: kgsl: Fix overflow issue to gpu_busy

gpu_busy is an unsigned 64 bit int and accumulates overtime
while running gpu intensive application(no slumber)
which eventually results in overflow.

Change-Id: Ifda376918ac58fa393a1958101d3d4b161bdd3c0
Signed-off-by: Nitheesh Muthuraj <nmuthuraj@codeaurora.org>