drm/exynos: restored kernel_fb_list when reiniting fb_helper

during recreating exynos_drm_fbdev as a new display device probes,
fb_helper is reinitialized but kernel fb is not changed
so kernel_fb_list should be restored after fb_helper is reinitialized.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>