drm/vma: Add a driver_private member to vma_node.

This allows drivers to distinguish between different types of vma_node's.
The readonly flag was unused and is thus removed.

This is a temporary solution, until i915 is converted completely to
use ttm for bo's.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Acked-by: Daniel Vetter <daniel@ffwll.ch> #irc
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210610070152.572423-4-thomas.hellstrom@linux.intel.com