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

drm/panfrost: 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

v3:
- No changes

v4:
- Fix the map_wc test in panfrost_ioctl_query_bo_info()

v5:
- Drop Steve's R-b (enough has changed to justify a new review)

v6:
- Collect R-b

v7:
- No changes

v8:
- Fix double drm_gem_object_funcs::export assignment

Signed-off-by: Faith Ekstrand <faith.ekstrand@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Link: https://patch.msgid.link/20251208100841.730527-13-boris.brezillon@collabora.com
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>