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.com
Reviewed-by: Sean Paul <sean@poorly.run>