drm/fb-helper: Fixup fill_info cleanup

I forgot the !CONFIG_FBDEV case. Also some kerneldoc needed more
adjusting.

Cc: Noralf Trønnes <noralf@tronnes.org>
Cc: Alex Deucher <alexander.deucher@amd.com>
Reported-by: kbuild test robot <lkp@intel.com>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190327125819.16478-1-daniel.vetter@ffwll.ch