drm/v3d: Fix debugfs reads of MMU regs.

They're in the hub, not the individual cores.

Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20190419001014.23579-1-eric@anholt.net


Reviewed-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>