drm/exynos: use exynos_dsi as drvdata

Use the exynos_dsi as drvdata instead of the encoder to further decouple
the driver from the encoder.

Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>