panfrost: Make ctx->job useful

panfrost: Make ctx->job useful

ctx->job is supposed to serve as a cache to avoid an hash table lookup
everytime we access the job attached to the currently bound FB, except
it was never assigned to anything but NULL.

Fix that by adding the missing assignment in panfrost_get_job_for_fbo().
Also add a missing NULL assignment in the ->set_framebuffer_state()
path.

While at it, add extra assert()s to make sure ctx->job is consistent.

Fixes: 59c9623d0a75 ("panfrost: Import job data structures from v3d")
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>