drm/cirrus: Fix cirrus drm driver for fbdev + qemu

Xorg fbdev driver requires smem_start/smem_len, otherwise
it tries to map 0 bytes as video memory.

Bugzilla: https://bugzilla.novell.com/show_bug.cgi?id=856760


Signed-off-by: Martin Koegler <martin.koegler@chello.at>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Dave Airlie <airlied@redhat.com>