drm/xe: Don't overmap identity VRAM mapping

In the Linux kernel, the following vulnerability has been resolved:

drm/xe: Don't overmap identity VRAM mapping

Overmapping the identity VRAM mapping is triggering hardware bugs on certain platforms. Use 2M pages for the last unaligned (to 1G) VRAM chunk.

v2: - Always use 2M pages for last chunk (Fei Yang) - break loop when 2M pages are used - Add assert for usable_size being 2M aligned v3: - Fix checkpatch