drm/exynos: fbdev: Remove offset into screen_buffer

drm/exynos: fbdev: Remove offset into screen_buffer

The screen_buffer field in struct fb_info contains the kernel address
of the first byte of framebuffer memory. Do not add the display offset.
This offset only describes scrolling during scanout.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Fixes: 19c8b8343d9c ("drm/exynos: fixed overlay data updating.")
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>
Cc: dri-devel@lists.freedesktop.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-samsung-soc@vger.kernel.org
Cc: <stable@vger.kernel.org> # v3.2+