drm/exynos/fbdev: don't set mode_config.fb_base

AFAICT, the fb_base of a drm_device's mode_config is never used.  It isn't
accessed by core drm, it isn't used by fbmem, and it isn't exposed to user
space.

Furthermore, it is probably supposed to be a physical address, not the
dma address mapped to the display controller, so this is just wrong.

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>