drm/panthor: Split panthor_vm_prepare_map_op_ctx() to prepare for reclaim

drm/panthor: Split panthor_vm_prepare_map_op_ctx() to prepare for reclaim

We're gonna need just the page table reservation logic when we restore
evicted BO mappings, so let's prepare for that by extracting the
op_ctx init and page table pre-allocation into separate helpers.

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/20260401134854.2275433-9-boris.brezillon@collabora.com
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>