drm/exynos: Fix de-registration ordering

'exynos_drm_pdev' was not getting unregistered if platform_driver_register()
failed. Fix the ordering to allow this. This also fixes the below warning by
moving the #endif macro. While at it also fix the ordering in the exit function
so that de-registration happens in opposite order of registration.
drivers/gpu/drm/exynos/exynos_drm_drv.c:768:1: warning: label
'err_unregister_pd' defined but not used [-Wunused-label]

Signed-off-by: Sachin Kamat <sachin.kamat@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>