The normal minimum for 12 ranks in a skill is 9th level. With a bard performing inspire greatness, you have two bonus HD, which are included as part of your character level, so when you would otherwise be 7th level, inspire greatness can get you to count as character level 9th, and thus allow you to get 12 skill ranks. Normally, this would not matter—you can’t change your skill ranks until you level up, and inspire greatness changes your character level, and thus your XP needed to level up. Get around that by having a psion provide psychic reformation during the inspire greatness—then you can rearrange your skill points immediately.
That’s still only 7th. Getting more ranks after that is somewhat harder—luckily we only need one. There are two options here:
If the bard performing inspire greatness has the Song of the Heart feat from Eberron Campaign Seeing, then their inspire greatness offers 3 bonus HD, and that solves that problem.
If a warlock 6th takes Cityscape’s Primary Contact as their 6th-level feat, they can get a “bonus rank” in one skill associated with the guild in which they have the Favored feat. This requires, of course, the Favored feat, as well as a guild you can join that’s associated with Knowledge (the planes). Cityscape lists several generic types of guilds, and adventurers’ guilds are listed as typically having it, for what it’s worth (and religious guilds can have it and scholastic guilds are even fairly likely to).
With thanks to HeyICanChan for pointing them out, Yenejg Togan’s planar syllabus from Dungeon vol. 88 and/or the magic volume found among the jackalweres’ treasure in Expedition to the Demonweb Pits could be used, as each of these grants a rank of Knowledge (the planes). Many DMs will find the permanent bonuses of Primary Contact, Yenejg Togan’s planar syllabus, and the jackalwere’s magical volume more palatable than inspire greatness abuse, but it may be extremely challenging for your average 6th-level warlock to obtain loot from an Encounter Level 10 fight from a specific adventure you may not even be playing.
That gets us to 6th, which is the earliest that a warlock can learn brimstone blast or hellrime blast, the invocations that hellfire warlock requires. That’s just about the best that can be done, because there’s no other way to get those invocations than to be a warlock 6th.