gpu: nvgpu: increase delays in do_idle()

gpu: nvgpu: increase delays in do_idle()

Increase the wait delays in do_idle() to 2000 mS and make use
of msleep instead of mdelays

Also, to check if GPU is rail gated or not, add a do-while()
loop which will keep checking the status and bail out as soon
as GPU is rail gated

This increase in delays is required to allow GPU sufficient
time to complete its work and get rail gated

These delays are specially needed during stress testing where
it is possible that a large amount of GPU work is blocked
during do_idle() and then it might take more time to complete
it while next do_idle() is waiting for it

Also, remove waiting on API gk20a_wait_channel_idle() for each
channels since it is sufficient to wait for refcount to be 1

bug 1529160

Change-Id: Ie541485fbdda76d79ae4a75dda928da240fc5d8f
Signed-off-by: Deepak Nibade <dnibade@nvidia.com>
Reviewed-on: http://git-master/r/434192
GVS: Gerrit_Virtual_Submit
Reviewed-by: Arto Merilainen <amerilainen@nvidia.com>
Reviewed-by: Sachin Nikam <snikam@nvidia.com>