drm/exynos: Fix potential NULL pointer dereference in suspend/resume paths

The patch 6e8edf8a: "drm/exynos: Fix suspend/resume support" introduced
a new code in suspend/resume paths. However it unconditionally dereference
drm_dev pointer, which might be NULL if suspend/resume happens before
Exynos DRM driver components bind. This patch fixes this issue.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: 6e8edf8a "drm/exynos: Fix suspend/resume support"
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>