drm/exynos: fix missed calling of drm_prime_gem_destroy()

When obj->import_attach is existed, code calling drm_prime_gem_destroy()
was removed from commit 67e93c80 ("drm/exynos: stop copying sg
table"), and it's a fault.

The drm_prime_gem_destroy() is cleanup function which GEM drivers need
to call when they use drm_gem_prime_import() to import dma-bufs, so
exynos-drm driver using drm_gem_prime_import() needs calling
drm_prime_gem_destroy().

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