msm: kgsl: Add a Kconfig setting for GPU idle timeout

Add a Kconfig option so that we can configure the
timeout for GPU to go into slumber per-environment
basis.

Change-Id: I36a2d78a4c987e635160db3056aab7b0d6af68a6
Signed-off-by: Urvashi Agrawal <urvaagra@codeaurora.org>