drm/panthor: Move panthor_gems_debugfs_init() to panthor_gem.c

drm/panthor: Move panthor_gems_debugfs_init() to panthor_gem.c

There's no reason for panthor_drv to know about panthor_gem.c internals,
so let's move the GEM debugfs init logic to panthor_gem.c.

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/20260401134854.2275433-4-boris.brezillon@collabora.com
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>