drm: Merge drm_info.c into drm_debugfs.c

Ever since

commit cb6458f9
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Thu Aug 8 15:41:34 2013 +0200

    drm: remove procfs code, take 2

Having the code shared between procfs and debugfs in the separate
drm_info.c file stopped making sense. Merge them.

Noticed because Lyude asked some questions on irc about why we even
have drm_info_node and I remember this old story.

Cc: Lyude Paul <lyude@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181121213510.31260-1-daniel.vetter@ffwll.ch