drm/i915: remove unused macro I915_GTT_OFFSET_NONE

Apparently the last user of the macro was removed in commit 9c4ce97d
("drm/i915/display: Be explicit in handling the preallocated vma").

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/b28b183457cf8871b1b070cbbbc3473bd5288210.1662390010.git.jani.nikula@intel.com