drm: Fix build when FBDEV_EMULATION is disabled

Commit be7f735cd5ea ("drm: Rely on mode_config data for fb_helper
initialization") broke the build when CONFIG_DRM_FBDEV_EMULATION is
disabled because it didn't update the prototype for drm_fb_helper_init
in that case.

Fixes: be7f735cd5ea ("drm: Rely on mode_config data for fb_helper
 initialization")
Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170202193900.22075-1-krisman@collabora.co.uk