drm/nouveau: disallow fbcon accel if running in interrupt context

A future commit will add locking to the DRM's channel, and there's numerous
problems that come up if we allow printk from an interrupt context to be
accelerated.  It seems saner to just disallow it completely.

As a nice side-effect, all the "to accel or not to accel" logic gets moved
out of the chipset-specific code.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nouveau_fbcon.h

@@ -40,13 +40,13 @@ struct nouveau_fbdev {
 
 void nouveau_fbcon_restore(void);
 
-void nv04_fbcon_copyarea(struct fb_info *info, const struct fb_copyarea *region);
-void nv04_fbcon_fillrect(struct fb_info *info, const struct fb_fillrect *rect);
-void nv04_fbcon_imageblit(struct fb_info *info, const struct fb_image *image);
+int nv04_fbcon_copyarea(struct fb_info *info, const struct fb_copyarea *region);
+int nv04_fbcon_fillrect(struct fb_info *info, const struct fb_fillrect *rect);
+int nv04_fbcon_imageblit(struct fb_info *info, const struct fb_image *image);
 int nv04_fbcon_accel_init(struct fb_info *info);
-void nv50_fbcon_fillrect(struct fb_info *info, const struct fb_fillrect *rect);
-void nv50_fbcon_copyarea(struct fb_info *info, const struct fb_copyarea *region);
-void nv50_fbcon_imageblit(struct fb_info *info, const struct fb_image *image);
+int nv50_fbcon_fillrect(struct fb_info *info, const struct fb_fillrect *rect);
+int nv50_fbcon_copyarea(struct fb_info *info, const struct fb_copyarea *region);
+int nv50_fbcon_imageblit(struct fb_info *info, const struct fb_image *image);
 int nv50_fbcon_accel_init(struct fb_info *info);
 
 void nouveau_fbcon_gpu_lockup(struct fb_info *info);