drm/fb-helper: Include prototype for drm_fb_helper_modinit()

Sparse complains:
drivers/gpu/drm/drm_fb_helper.c:2409:12: warning: symbol 'drm_fb_helper_modinit' was not declared. Should it be static?

Include the header with the correct prototype.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190710125143.9965-1-ville.syrjala@linux.intel.comReviewed-by: Sean Paul <sean@poorly.run>

drivers/gpu/drm/drm_fb_helper.c

@@ -46,6 +46,7 @@
 #include <drm/drm_print.h>
 #include <drm/drm_vblank.h>
 
+#include "drm_crtc_helper_internal.h"
 #include "drm_internal.h"
 
 static bool drm_fbdev_emulation = true;