drm/msm/hdmi: Point to the right struct device

DRM device's dev (hdmi->dev->dev) points to the mdss_mdp device
handle. Instead, we should get a reference to the mdss_hdmi
handle.

Signed-off-by: Stephane Viau <sviau@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>