drm/exynos: s/exynos_gem_obj/obj in exynos_drm_fbdev.c

The variable name "exynos_gem_obj" is too long, so some lines exceed 80
characters. It's simple to use "obj" instead of "exynos_gem_obj".

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Inki Dae <inki.dae@samsung.com>