drm/exynos: add iommu support for hdmi driver

Changelog v2:
move iommu support feature to mixer side.
And below is Prathyush's comment.

According to the new IOMMU framework for exynos sysmmus,
the owner of the sysmmu-tv is mixer (which is the actual
device that does DMA) and not hdmi.
The mmu-master in sysmmu-tv node is set as below in exynos5250.dtsi
	sysmmu-tv {
		-
		mmu-master = <&mixer>;
	};

Changelog v1:
The iommu will be enabled when hdmi sub driver is probed and
will be disabled when removed.

Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
diff --git a/drivers/gpu/drm/exynos/exynos_hdmi.c b/drivers/gpu/drm/exynos/exynos_hdmi.c
index 2c115f8..c73f438 100644
--- a/drivers/gpu/drm/exynos/exynos_hdmi.c
+++ b/drivers/gpu/drm/exynos/exynos_hdmi.c
@@ -74,6 +74,7 @@
 	struct mutex			hdmi_mutex;
 
 	void __iomem			*regs;
+	void				*parent_ctx;
 	int				external_irq;
 	int				internal_irq;
 
@@ -84,7 +85,6 @@
 	int cur_conf;
 
 	struct hdmi_resources		res;
-	void				*parent_ctx;
 
 	int				hpd_gpio;