nvidia fbdev: fix powerpc xmon scribbles

xmon writes garbage on the screen because the nvidia console driver has
changed the line pitch from what the firmware set it to.  Fix it by making
the nvidia driver inform the btext engine (which xmon uses if the screen is
its output device) about changes to display resolution.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Adrian Bunk <bunk@stusta.de>