so I got this working. my pin mappings were wrong. they should be the following:
In 18 bit mode (note: kernel command line should be something on the order of video=mxcfb0:dev=lcd,800x480M@55,if=RGB666):
DISP0_DAT0:DISP0_DAT5 - B0-B5
DISP0_DAT6:DISP0_DAT11 - G0-G5
DISP0_DAT12:DISP0_DAT17 - R0-R5
In 16 bit mode:
DISP0_DAT0:DISP0_DAT4 - B0-B4
DISP0_DAT5:DISP0_DAT10 - G0-G5
DISP0_DAT11:DISP0_DAT15 - R0-R4
In the IMX6 reference manual, there is mention that this mapping can be changed in software (see sec 37.4.7.5.1 Bus Mapping Unit) but I haven't given this a try since I didn't need to.