drm/i915/dg1: remove __maybe_unused leftover

This was added in commit 05e26584 ("drm/i915/dg1: add initial DG-1
definitions") so we could continue to add support for DG1 without
risk to expose a broken UAPI. Now that we added DG1 to the PCI ID list
i915 may bind to, remove the leftover.

Fixes: d5ef86b3

 ("drm/i915: Add pci ids and uapi for DG1")
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210819210349.95103-1-lucas.demarchi@intel.com