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
f9aa76a8 ("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.comSigned-off-by: Gerd Hoffmann <kraxel@redhat.com>

drivers/gpu/drm/cirrus/cirrus_fbdev.c

@@ -169,7 +169,6 @@ static int cirrusfb_create(struct drm_fb_helper *helper,
 	struct drm_mode_fb_cmd2 mode_cmd;
 	void *sysram;
 	struct drm_gem_object *gobj = NULL;
-	struct cirrus_bo *bo = NULL;
 	int size, ret;
 
 	mode_cmd.width = sizes->surface_width;
@@ -185,8 +184,6 @@ static int cirrusfb_create(struct drm_fb_helper *helper,
 		return ret;
 	}
 
-	bo = gem_to_cirrus_bo(gobj);
-
 	sysram = vmalloc(size);
 	if (!sysram)
 		return -ENOMEM;