drm: use "0" instead of "" for deprecated driver date

libdrm does not like the empty string for driver date. Use "0" instead,
which has been used by virtio previously.

Reported-by: Steven Price <steven.price@arm.com>
Closes: https://lore.kernel.org/r/9d0cff47-308e-4b11-a9f3-4157dc26b6fa@arm.com
Fixes: 7fb8af67

 ("drm: deprecate driver date")
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240510090951.3398882-1-jani.nikula@intel.com
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Tested-by: Steven Price <steven.price@arm.com>
Reviewed-by: Simon Ser <contact@emersion.fr>