I did finished a project a few weeks before.
In fact I was asking myself the same question and have examined the datasheets LCD-Controller and DMA Controller Section. But then We decided to use an OS, it was cheaper (faster) than writing his own LCD interface.
You have the choice between Linux or WinCE. If you don't want to use an OS look at U-Boot as well.
Please tell me which LCD did you use. PXA270 cannot directly interface to 24 bit LCD. For 24 bit colors one has to use a Smart LCD's which have their own frame buffer.
yes as I remember there is only 18bit wide LCD bus coming out from the PXA.
The LCD was 24 bit, so we connected the LSB bits of each color to the ground.
We chose Sharp. There are 2 kind of LCDs available : one that required extra timing signals which the PXA don't generate (you have to place a small CPLD between) and the other can be directly interfaced with VGA from the PXA requiring just some line drivers. Look at the LQ043XXX series from Sharp.