drm/exynos: fix double call of drm_prime_(init/destroy)_file_private

The double invocations are incorrect but seem to be safe so I don't
think this will fix any bugs.

Before:

[    7.639366] drm_prime_init_file ee3675d0
[    7.639377] drm_prime_init_file ee3675d0
[    7.639507] drm_prime_destroy_file ee3675d0
[    7.639518] drm_prime_destroy_file ee3675d0
[    7.639802] drm_prime_init_file ee372390
[    7.639810] drm_prime_init_file ee372390
[    8.473316] drm_prime_init_file ee356390
[    8.473331] drm_prime_init_file ee356390

After:

[    6.363842] drm_prime_init_file edc2e5d0
[    6.363994] drm_prime_destroy_file edc2e5d0
[    6.364260] drm_prime_init_file edc2e750
[    8.004837] drm_prime_init_file ee36ded0

Signed-off-by: Mandeep Singh Baines <msb@chromium.org>
Acked-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>