drm/exynos: reorder framebuffer init sequence

For user framebuffers it's easier to just inline the
exynos_drm_framebuffer_init helper instead of trying to adjust it -
most of the things that helper sets up need to be overwritten anyway
again due to the multiple backing storage objects support exynos has,
but does not use for the fbdev.

Changelog v2:
- fix NULL point issue to first gem object of exynos drm framebuffer.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Inki Dae <inki.dae@samsung.com>