I think the licensing mostly applies for chip manufacturers that make ICs with built in MIPI.
There are examples on the Lattice site of doing MIPI with MachXO2 (And i thing a few others). But its not all open source. The actual MIPI part of it is provided as a binary blob, but it does the hard parts of MIPI for you of driving the IO pins and synchronizing itself to the packets on the bus. For my use with a raspberry pi i found that the provided demo code didn't quite work so i rewrote some of the data parsing, but the part that turns MIPI into a convenient 8 bit parallel bus worked great.
You have to be part of the MIPI group to get the full spec, but i could still find enough information online to be able to know what every byte sent over the bus meant.
Also if you are looking to drive a LCD you likely also need the LCD documentation for it or at the very least a product that runs the LCD so you can sniff the data. This is because MIPI is not a completely intercompatible bus like HDMI where you plug it in and it simply works. The LCD might have specific timing requirements to make it happy or even require an initialization sequence. Much like some parallel RGB bus LCDs need to be initialized and configured over I2C or SPI these MIPI displays can have similar internal registers that sometimes have to be written to using specific MIPI commands. Timings can be found out by trial and error, but whole initialization sequences no way.