drm/exynos: g2d: use autosuspend mode for PM runtime

The runqueue worker currently issues a get() when a new
node is processed, and a put() once a node is completed.

The corresponding suspend and resume calls currently only
do clock gating, but with the upcoming introduction of
IOMMU runpm also the corresponding IOMMU domain gets
enabled (for get()) and disabled (for put()). This
introduces performance regressions with we mitigate here.

Switch PM runtime to autosuspend, such that clock gating
and IOMMU control only happens when the engine is idle for
a 'long' time.

Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>