drm/i915: Add intel_modeset_probe_defer() helper

The upcoming privacy-screen support adds another check for
deferring probe till some other drivers have bound first.

Factor out the current vga_switcheroo_client_probe_defer() check
into an intel_modeset_probe_defer() helper, so that further
probe-deferral checks can be added there.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211005202322.700909-10-hdegoede@redhat.com