drm/i915: Fix doclinks

Update locations for

./drivers/gpu/drm/i915/i915_vma.h:1: warning: 'Virtual Memory Address' not found
./drivers/gpu/drm/i915/i915_gem_gtt.c:1: warning: 'Global GTT views' not found

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200302145254.520447-1-chris@chris-wilson.co.uk