drm/panthor: Add flag to map GEM object Write-Back Cacheable

drm/panthor: Add flag to map GEM object Write-Back Cacheable

Will be used by the UMD to optimize CPU accesses to buffers
that are frequently read by the CPU, or on which the access
pattern makes non-cacheable mappings inefficient.

Mapping buffers CPU-cached implies taking care of the CPU
cache maintenance in the UMD, unless the GPU is IO coherent.

v2:
- Add more to the commit message
- Tweak the doc
- Make sure we sync the section of the BO pointing to the CS
  syncobj before we read its seqno

v3:
- Fix formatting/spelling issues

v4:
- Add Steve's R-b

v5:
- Drop Steve's R-b (changes in the ioctl semantics requiring
  new review)

v6:
- Fix the uAPI doc
- Fix inverted logic in some comment

v7:
- No changes

v8:
- Collect R-b

Signed-off-by: Loïc Molinari <loic.molinari@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Link: https://patch.msgid.link/20251208100841.730527-7-boris.brezillon@collabora.com
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>