drm/exynos: mixer: Fix enabling of the runtime power management

Runtime power management is essential for the Exynos Mixer driver
operation. It should be enabled before adding its DRM component, because
in some cases (when deferred probe takes place due to the IOMMU
availability) the DRM driver might be initialized directly from the
Mixer's component_add() call, what results in starting the driver
operation without enabling the runtime power management.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>