gpu: ion: Clarify variable names and comments around heap ids v types

There is some confusion between when to use the heap type and when
the id.  This patch clarifies this by using clearer variable names
and describing the intention in the comments.  Also fixes the client
debug code to print heaps by id instead of type.

Change-Id: I63f58a5336666ddb269147c742d804d28326acd0