drm: deprecate driver date

The driver date serves no useful purpose, because it's hardly ever
updated. The information is misleading at best.

As described in Documentation/gpu/drm-internals.rst:

  The driver date, formatted as YYYYMMDD, is meant to identify the date
  of the latest modification to the driver. However, as most drivers
  fail to update it, its value is mostly useless. The DRM core prints it
  to the kernel log at initialization time and passes it to userspace
  through the DRM_IOCTL_VERSION ioctl.

Stop printing the driver date at init, and start returning the empty
string "" as driver date through the DRM_IOCTL_VERSION ioctl.

The driver date initialization in drivers and the struct drm_driver date
member can be removed in follow-up.

Reviewed-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Acked-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240429164336.1406480-1-jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>