drm/xe: NULL binding implementation

Add uAPI and implementation for NULL bindings. A NULL binding is defined
as writes dropped and read zero. A single bit in the uAPI has been added
which results in a single bit in the PTEs being set.

NULL bindings are intendedd to be used to implement VK sparse bindings,
in particular residencyNonResidentStrict property.

v2: Fix BUG_ON shown in VK testing, fix check patch warning, fix
xe_pt_scan_64K, update __gen8_pte_encode to understand NULL bindings,
remove else if vma_addr

Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Suggested-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>