drm: Add a helper function for printing a debugfs_regset32.

The debugfs_regset32 is nice to use for reducing boilerplate in
dumping a bunch of regs in debugfs, but we also want to be able to
print to dmesg them at runtime for driver debugging.  drm_printer lets
us format debugfs and the printk the same way.

v2: Add some kerneldoc for the function (requested by danvet)

Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20190220210343.28157-1-eric@anholt.net
Reviewed-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com> (v1)
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>