drm/fb-helper: Fix sparse warnings

drm/drm_fb_helper.c:2306:12: warning: symbol 'drm_fb_helper_modinit' was not declared. Should it be static?

While at it, move the lefover static inline to the right place.

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1474292035-15695-4-git-send-email-ville.syrjala@linux.intel.com