drm/cirrus: Remove set but not used variable 'bo'

drm/cirrus: Remove set but not used variable 'bo'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/gpu/drm/cirrus/cirrus_fbdev.c: In function 'cirrusfb_create':
drivers/gpu/drm/cirrus/cirrus_fbdev.c:172:20: warning:
 variable 'bo' set but not used [-Wunused-but-set-variable]

It never used since introduction in commit
5724b1f4b360 ("drm/kms: driver for virtual cirrus under qemu")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1542283836-152176-1-git-send-email-yuehaibing@huawei.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>