tc: update GPU specific export variables for debian

* Removed the hardcoded GPU DDK export variables required to
  build GPU DDK from source for debian.
* Moved mali DDK source for debian under src/debian/mali. This
  is to differentiate debian mali source form android mali source
  when same TC build will be built both for android and debian.

Signed-off-by: Rupinderjit Singh <rupinderjit.singh@arm.com>