Building on DoubleAA's first point, we find that with King David, when Shimi cursed him (II Sam. 16:5-8), David let it slide not only at the time (ibid. vv. 10-12), but even after he had been reinstated as king (ibid. 19:23-24). The Mishneh Lamelech (Parshas Derachim, derush 11) explains that David was of the opinion that during Avshalom's rebellion he had in fact lost his kingly status (the same view is also recorded in the Yerushalmi, Horios 3:2), so he was entitled to forgive the slight to his honor.
The same thing, then, would apply here, and even more so: when the brothers had harmed him, Yosef was far from being a ruler yet.
Arguably, too (even without considering the question of whether he was a Jewish king), there is the simple fact that a king is defined in halachah - in connection with what kind of animal he brings as a korban chatas for an inadvertent sin - as someone who has no superior other than Hashem Himself (Horios 10a and 11b, and from there in Rambam, Hil. Shegagos 15:9). Yosef, on the other hand, was subordinate to Pharaoh - רק הכסא אגדל ממך.
[Although this isn't really dispositive, since (a) the definition of a king might be different regarding shegagos vs. honor, and (b) R. Yehudah Hanassi asks there whether he would be considered a king for purposes of shegagos (כגון אני מהו בשעיר), and R. Chiya rejects that possibility only because he's subordinate to the Reish Galusa in Babylonia - but doesn't take into consideration the fact that Rebbi is also a subject of the Roman emperor. So perhaps indeed "a king" regarding shegagos is defined as the highest Jewish authority, without considering any non-Jewish overlordship.]