drm/exynos: remove exynos_mixer.h

The contents of exynos_mixer.h are used only in exynos_mixer.c, so
separated header is unnecessary.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>