drm/i915: Allow switching away via vga-switcheroo if uninitialized

Always allow switching away via vga-switcheroo if the display is
uninitalized. Instead prevent switching to i915 if the device has
not been initialized.

This issue was introduced by commit 5df7bd13 ("drm/i915: skip
display initialization when there is no display") protected, which
protects code paths from being executed on uninitialized devices.
In the case of vga-switcheroo, we want to allow a switch away from
i915's device. So run vga_switcheroo_process_delayed_switch() and
test in the switcheroo callbacks if the i915 device is available.

Fixes: 5df7bd13 ("drm/i915: skip display initialization when there is no display")
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Cc: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: José Roberto de Souza <jose.souza@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: "Ville Syrjälä" <ville.syrjala@linux.intel.com>
Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Cc: "Jouni Högander" <jouni.hogander@intel.com>
Cc: Uma Shankar <uma.shankar@intel.com>
Cc: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Cc: "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Ramalingam C <ramalingam.c@intel.com>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Andi Shyti <andi.shyti@linux.intel.com>
Cc: Andrzej Hajda <andrzej.hajda@intel.com>
Cc: "José Roberto de Souza" <jose.souza@intel.com>
Cc: Julia Lawall <Julia.Lawall@inria.fr>
Cc: intel-gfx@lists.freedesktop.org
Cc: <stable@vger.kernel.org> # v5.14+
Link: https://patchwork.freedesktop.org/patch/msgid/20230116115425.13484-2-tzimmermann@suse.de