drm/v3d: Dump V3D error debug registers in debugfs, and one at reset.

Looking at a hang recently, I noticed these registers that might tell
me if something obvious was wrong.  They didn't help in this case, but
keep it around for the future.

Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20190419001014.23579-3-eric@anholt.net


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