drm/exynos: Remove exynos_gem from struct exynos_drm_fbdev

Fbdev's framebuffer stores a pointer to the GEM object. Remove
struct exynos_drm_fbdev.exynos_gem, which contains the same value.
No functional changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>