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: Ie8b3dadded52e18590fcb2ca94001f6ed46ef07d
Signed-off-by: Rebecca Schultz Zavin <rebecca@android.com>
Git-commit: dbb80960d2f720c9d06dcbe4726fc5a1e2c4e8e0
Git-repo: https://android.googlesource.com/kernel/common/
[mitchelh@codeaurora.org: Had to skip most of this patch since already
had most of it. Also had to apply to both ion.h files due to the UAPI
split. Also reverting 0b017a376827fc which renamed heap_id_mask to
heap_mask.]
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>