drm/vkms: use new debugfs device-centered functions

Replace the use of drm_debugfs_create_files() with the new
drm_debugfs_add_files() function, which centers the debugfs files
management on the drm_device instead of drm_minor. Moreover, remove the
debugfs_init hook and add the debugfs files directly on vkms_create(),
before drm_dev_register().

Signed-off-by: Maíra Canal <mcanal@igalia.com>
Reviewed-by: Melissa Wen <mwen@igalia.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Maíra Canal <mairacanal@riseup.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20221219120621.15086-7-mcanal@igalia.com