fb_copyarea: Add DMA-based fill support

Uses bcm2835-specific ioctl to accelerate large area
fills using DMA.

Signed-off-by: Luke Diamand <luke@diamand.org>
fb_copyarea: make thresholds configurable

This change adds support for configuring the DMA copy and
fill thresholds via the X server configuration file.

Signed-off-by: Luke Diamand <luke@diamand.org>