First: Run the xf86config program to create a correct configuration.
You can turn off hardware cursor by inserting the following line in the Device section of the XF86Config file:
Or turn off hardware acceleration:
Or turn off MMIO
Do not specify either the amount of video RAM or which chipset in the config file. It is better to let the driver probe for this. Also don't put any "clocks" line in the device section since these chips have a fully programmable clock that can almost take any modeline you throw at it. It might though fail at some specific clock values but in that case you should just try a slightly different clock and it should work.