gralloc: Fix a missed unmap.

Change I86300a1b4e3008f9d0884940420e9036c47a016f introduces a
regression by mapping metadata part of secure buffers, but not
unmapping that. This change fixes the map/unmap asymmetry.

Change-Id: I078eef1139642bd6d8a01145ab1c81060621fb8d
1 file changed