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

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Fei Yang <fei.yang@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Fei Yang <fei.yang@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240603181824.1927675-1-matthew.brost@intel.com