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

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

drivers/gpu/drm/ast/ast_fb.c: In function 'astfb_create':
drivers/gpu/drm/ast/ast_fb.c:194:17: warning:
 variable 'bo' set but not used [-Wunused-but-set-variable]

It never used since introduction in commit 312fec14 ("drm: Initial KMS
driver for AST (ASpeed Technologies) 2000 series (v2)")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1544258185-50430-1-git-send-email-yuehaibing@huawei.com

drivers/gpu/drm/ast/ast_fb.c

@@ -191,7 +191,6 @@ static int astfb_create(struct drm_fb_helper *helper,
 	int size, ret;
 	void *sysram;
 	struct drm_gem_object *gobj = NULL;
-	struct ast_bo *bo = NULL;
 	mode_cmd.width = sizes->surface_width;
 	mode_cmd.height = sizes->surface_height;
 	mode_cmd.pitches[0] = mode_cmd.width * ((sizes->surface_bpp + 7)/8);
@@ -206,7 +205,6 @@ static int astfb_create(struct drm_fb_helper *helper,
 		DRM_ERROR("failed to create fbcon backing object %d\n", ret);
 		return ret;
 	}
-	bo = gem_to_ast_bo(gobj);
 
 	sysram = vmalloc(size);
 	if (!sysram)