drm/exynos: added mode_fixup feature and code clean.

this patch adds mode_fixup feature for hdmi module that
specific driver changes current mode to driver desired mode
properly.

Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>