msm: kgsl: Use increasing delays to transition to NAP

When active count goes down to zero, the GPU is about
to be idle.  Wait for it to go fully idle and transition
to the NAP state.

Change-Id: I06ed7bc07125e757ae2d665b2fd4135a6e0296fe
CRs-Fixed: 436977
Signed-off-by: Lucille Sylvester <lsylvest@codeaurora.org>