drm/vc4: vc4_debugfs_regset32() can be static

Fixes: c9be804c ("drm/vc4: Use common helpers for debugfs setup by the driver components.")
Signed-off-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20190403213629.GA31570@lkp-ib03