libkms/exynos: fix memory leak in error path

This patch fixes memory leak in error path of exynos_bo_create().

Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>