drm/fb-helper: Unexport fill_{var,info}

Not used by drivers anymore.

v2: Rebase

Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190326132008.11781-21-daniel.vetter@ffwll.ch