gpu: ion: Add debug information for individual heaps

Currently, the debugfs information provides aggregated data
for all carveout heaps. This isn't particularly helpful for
determining allocation problems with individual carveout heaps.
Fix this to give data for individual carveout heaps.

Change-Id: I60a10851052412e20c40f1862d6dcc22093fd982
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>