drm/exynos: remove unnecessary runtime pm operations

In booting, we can see a below message.

[    3.241728] exynos-mixer 14450000.mixer: Unbalanced pm_runtime_enable!

Already pm_runtime_enable is called by probe function. Remove
pm_runtime_enable/disable from mixer_bind and mixer_unbind.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>