drm: add drm_print_bits

New helper to print named bits of some value (think flags fields).

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: http://patchwork.freedesktop.org/patch/msgid/20190904054740.20817-2-kraxel@redhat.com