msm: kgsl: Properly set fault bit in adreno_set_gpu_fault

Replacing atomic_add by atomic_or to set the fault bit
properly without overwriting other bits.

Change-Id: Ie25d8e5f32d5b1f0d6e50931cc23bd32b2f01b6d
Signed-off-by: Pankaj Gupta <gpankaj@codeaurora.org>