drm/panthor: Group panthor_kernel_bo_xxx() helpers

drm/panthor: Group panthor_kernel_bo_xxx() helpers

Move all panthor_kernel_bo_xxx() helpers at the end of the file, just
before the debugfs init logic. This will make further panthor_gem.c
refactoring more readable.

v2:
- Collect R-bs

v3:
- No changes

v4:
- No changes

v5:
- No changes

v6:
- No changes

v7:
- No changes

Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Link: https://patch.msgid.link/20260401134854.2275433-5-boris.brezillon@collabora.com
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>