drm/nouveau: add missing header dependencies

We get a few warnings when building kernel with W=1:
drivers/gpu/drm/nouveau/nvkm/core/firmware.c:34:1: warning: no previous prototype for 'nvkm_firmware_get' [-Wmissing-prototypes]
drivers/gpu/drm/nouveau/nvkm/core/firmware.c:58:1: warning: no previous prototype for 'nvkm_firmware_put' [-Wmissing-prototypes]
drivers/gpu/drm/nouveau/nvkm/subdev/fb/sddr3.c:69:1: warning: no previous prototype for 'nvkm_sddr3_calc' [-Wmissing-prototypes]
drivers/gpu/drm/nouveau/nvkm/subdev/fb/sddr2.c:60:1: warning: no previous prototype for 'nvkm_sddr2_calc' [-Wmissing-prototypes]
....

In fact, these functions are declared in
drivers/gpu/drm/nouveau/include/nvkm/core/firmware.h
drivers/gpu/drm/nouveau/nvkm/subdev/fb/ram.h
drivers/gpu/drm/nouveau/nvkm/subdev/volt/priv.h
drivers/gpu/drm/nouveau/nvkm/engine/gr/nv50.h
drivers/gpu/drm/nouveau/dispnv04/disp.h.
So this patch adds missing header dependencies.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/dispnv04/overlay.c

@@ -33,7 +33,7 @@
 #include "nouveau_connector.h"
 #include "nouveau_display.h"
 #include "nvreg.h"
-
+#include "disp.h"
 
 struct nouveau_plane {
 	struct drm_plane base;