drm/i915: Disable mmap ioctl for gen12+

The platform should exclusively use mmap_offset, one less path to worry
about for discrete.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210601074654.3103-14-thomas.hellstrom@linux.intel.com