drm/i915/gt: Limit the reserved VM space to only the platforms that need it

Commit 9bb66c17 ("drm/i915: Reserve some kernel space per
vm") reduces the available VM space of one page in order to apply
Wa_16018031267 and Wa_16018063123.

This page was reserved indiscrimitely in all platforms even when
not needed. Limit it to DG2 onwards.

Fixes: 9bb66c17

 ("drm/i915: Reserve some kernel space per vm")
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Cc: Andrzej Hajda <andrzej.hajda@intel.com>
Cc: Chris Wilson <chris.p.wilson@linux.intel.com>
Cc: Jonathan Cavitt <jonathan.cavitt@intel.com>
Cc: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Acked-by: Michal Mrozek <michal.mrozek@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240327200546.640108-1-andi.shyti@linux.intel.com
(cherry picked from commit 97216344

)
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>