drm/exynos: add check for the device power status

V2: Add mutex protection, while read.

The hdmi and mixer win_commit calls currently are
not checking the status of IP before updating the
respective registers, this patch adds this check.

Signed-off-by: Shirish S <s.shirish@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>