gpu: ion: exynos: fix "error: double free of pages"

This patch fixes double free of 'pages' array when pfnmap_digger()
fails.

Change-Id: I25518b3bc758fee13143d67457b3d682b526d1e8
Reported-by: Jonghwan Choi <jhbird.choi@samsung.com>
Signed-off-by: KyongHo Cho <pullip.cho@samsung.com>