gpu_create_kernel: plug memory leak on error path
gpu_create_kernel: plug memory leak on error path

In particular, if the allocation of the isl_id referencing the ppcg_kernel
failed, then the ppcg_kernel does not get freed.
Free it explicitly.

Signed-off-by: Sven Verdoolaege <235c10dd23b819f81cdc9756a251746bc184cab6@gmail.com>

Meinersbur/ppcg,Meinersbur/ppcg