media: exynos: mfc: Do not change clocks if a DRM context is running

Turns out the previous patch did not handle all crash cases and we
need a more aggressive approach to this.

Change-Id: I855533212d407c7a9f67fc9add1185cd2cd580c0
(cherry picked from commit b79a6c3c4aa4ffcf06ba229ea4e89e37782d4328)