Hello,
How are you setting the device tree for the MIPI-PANEL?
To change it. Please make it as follows:
u-boot -> env edit fdt_file
u-boot -> fdt_file:fsl-imx8mq-evk-dcss-rm67191.dtb # or fsl-imx8mq-evk-dcss-rm67191-b3.dtb, that the depends of the revision of your board.
u-boot -> saveenv
u-boot -> boot
Whit that configuration you should be able to use the MIPI-Panel.
Hope it could solve your problem.
Best regards,
Diego.