drm/vc4: Make sure that the v3d ident debugfs has vc4's power on.

Otherwise, you sometimes decode the ident fields based on 0xdeadbeef
register reads.

Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20190220210343.28157-7-eric@anholt.net
Reviewed-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>