drm/exynos: remove unused codes in hdmi and mixer

Some members in struct mixer_context aren't used and the define
HDMI_OVERLAY_NUMBER is unused in hdmi driver, remove them.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>