drm/vkms: Create a debugfs file to get vkms config data

Creating a vkms_config debugfs file in vkms_drv.c to get/track vkms config
data, for the long-term plan of making vkms configurable and have multiple
different instances.

Reviewed-by: Melissa Wen <melissa.srw@gmail.com>
Signed-off-by: Beatriz Martins de Carvalho <martinsdecarvalhobeatriz@gmail.com>
Signed-off-by: Melissa Wen <melissa.srw@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210708092002.11847-1-martinsdecarvalhobeatriz@gmail.com