intel/aub_write: turn context images arrays into functions

intel/aub_write: turn context images arrays into functions

We'll make them more parameterized in a later commit.

As this is just a transitional commit, we allow ourself to leak the
context images allocated in get_context_init(). We'll fix this in the
next commit.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Rafael Antognolli <rafael.antognolli@intel.com>