gpu_array_tile: keep track of isl_ctx
gpu_array_tile: keep track of isl_ctx

This means we no longer need to pass along an isl_ctx to tile_size.

Signed-off-by: Sven Verdoolaege <e5350bbed4977f5eb8ae1dc6abd9ae59d21ace75@kotnet.org>

Meinersbur/ppcg,Meinersbur/ppcg,nicolasvasilache/ppcg,nicolasvasilache/ppcg