drm/exynos: Convert dp driver to use panel_ops

drm/exynos: Convert dp driver to use panel_ops

This patch changes the dp driver to implement panel_ops. This gets
rid of the dp_dev hack that was introduced when dp moved into drm.

BUG=chrome-os-partner:11158, chrome-os-partner:9855,
    chrome-os-partner:10717
TEST=Tested on snow, no regressions detected

Change-Id: Ia0e2939f635a2406a1ac29e172cb1ae59d13acbe
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/36701
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>