video: tegra: host: Simplify keepalive flag

video: tegra: host: Simplify keepalive flag

Currently nvhost uses pm_qos API for keeping certain devices
powered. However, due to API limitations this may lead to cases
where the device does not get powered immediately when setting
the flag - or the power domain remains powered after the constraint
has been removed.

This patch refactors code to use only pm_runtime for enabling or
disabling power-off functionality.

Change-Id: I2fc9dd2d4c62626da847d37d689b5fad1935a25e
Signed-off-by: Arto Merilainen <amerilainen@nvidia.com>
Reviewed-on: http://git-master/r/781704
Reviewed-by: Automatic_Commit_Validation_User
Reviewed-by: Deepak Nibade <dnibade@nvidia.com>
Reviewed-by: Shridhar Rasal <srasal@nvidia.com>