gpu: ion: Loop on the handle count when destroying

When destroying a handle, all kernel mappings to that handle
should be destroyed. Other handles may still have references
and valid mappings to the buffer underneath which should not
be destroyed. Loop on the handle reference count, not the buffer
reference count to get rid of all kernel mappings for the handle.

Change-Id: I7dc5d6a86513fc5fa4e21110ceab434714ea2493
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>