drm/vmwgfx: Fix atomic mode set check

vmw_kms_atomic_check_modeset() is currently checking config using the
legacy state, which is updated after a commit has happened.

This means vmw_kms_atomic_check_modeset() will reject an invalid config
on the next update rather than the current one.

Fix this by using the new states for config checking

Signed-off-by: Sinclair Yeh <syeh@vmware.com>
Reviewed-by: Deepak Rawat <drawat@vmware.com>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>