drm/exynos: exynos_drm.h header file fixes

First of all #ifdef __KERNEL__ was added to exynos_drm.h to
mark the part that should be left out of userspace.
Secondly exynos_drm.h was added to include/drm/Kbuild, so it
will be included when doing make headers_install.

Signed-off-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>