drm/panfrost: Bump the driver version to 1.6

drm/panfrost: Bump the driver version to 1.6

Bump the driver version to reflect the new cached-CPU mapping
capability.

v2:
- Quickly describe what the new version exposes in the commit message

v3:
- Add Steve's R-b

v4:
- No changes

v5:
- No changes

v6:
- No changes

v7:
- No changes

v8:
- No changes

Signed-off-by: Faith Ekstrand <faith.ekstrand@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Link: https://patch.msgid.link/20251208100841.730527-14-boris.brezillon@collabora.com
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>