drm/exynos/hdmi: add bridge support

On TM2/TM2e platforms HDMI output is connected to MHL bridge
SiI8620. To allow configure UltraHD modes on the bridge
and to eliminate unsupported modes this bridge should be
attached to drm_encoder implemented in exynos_hdmi.

Changelog v1:
- fix drm_attach_bridge argument.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>