staging: drm/imx: also allow to allocate only 2 DMFC slots for DP full plane

Connecting a 320x240 parallel display on i.MX6 resulted in an invalid DRDY
signal because the DC would not receive NL/EOL events on every line.
Reducing the allocated DMFC space from 4 slots (256 * 128-bit) to 2 slots
(128 * 128-bit) solved the problem.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>