drm/vkms: Create separate Kconfig file for VKMS

This aligns with most other DRM drivers and will allow
us to add new VKMS config options without polluting
the DRM Kconfig.

v3:
 - Change SPDX to GPL-2.0-only to match DRM KConfig
   SPDX (Simon)

Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Simon Ser <contact@emersion.fr>
Signed-off-by: Melissa Wen <melissa.srw@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231108163647.106853-4-harry.wentland@amd.com