omxbufferpool: fix memory mapping with offset

gst_memory_map() is already adding the offset to the mapped pointer.
Doing it in the memory implementation was resulting in the offset being
accounted twice.

It doesn't matter yet as we are only creating memory without offset for
now but it will once we'll start sharing OMX memories.
4 jobs for fix-map in 7 minutes and 49 seconds (queued for 1 second)