drm/vkms: Fix use after free and double free on init error

drm/vkms: Fix use after free and double free on init error

If the driver initialization fails, the vkms_exit() function might
access an uninitialized or freed default_config pointer and it might
double free it.

Fix both possible errors by initializing default_config only when the
driver initialization succeeded.

Reported-by: Louis Chauvet <louis.chauvet@bootlin.com>
Closes: https://lore.kernel.org/all/Z5uDHcCmAwiTsGte@louis-chauvet-laptop/
Fixes: 2df7af93fdad ("drm/vkms: Add vkms_config type")
Signed-off-by: José Expósito <jose.exposito89@gmail.com>
Reviewed-by: Thomas Zimmermann <tzimmremann@suse.de>
Reviewed-by: Louis Chauvet <louis.chauvet@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250212084912.3196-1-jose.exposito89@gmail.com
Signed-off-by: Louis Chauvet <louis.chauvet@bootlin.com>