drm/exynos: gem: Fix sparse warning

kvaddr element of the exynos_gem object points to a memory buffer, thus
it should not have a __iomem annotation. Then, to avoid a warning or
casting on assignment to fbi structure, the screen_buffer element of the
union should be used instead of the screen_base.

Reported-by: kernel test robot <lkp@intel.com>
Suggested-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>