drm/panthor: Lazily allocate pages on mmap()

drm/panthor: Lazily allocate pages on mmap()

Defer pages allocation until their first access.

v2:
- Don't deal with FAULT_FLAG_INTERRUPTIBLE
- Make sure bo->backing.pages is never an ERR_PTR()
- Drop a useless vm_fault_t local var
- Fix comment in panthor_gem_fault()

v3:
- Collect R-bs

v4:
- No changes

v5:
- No changes

v6:
- Fix huge_fault handling

v7:
- No changes

Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Link: https://patch.msgid.link/20260401134854.2275433-8-boris.brezillon@collabora.com
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>