drm/pagemap_util: Ensure proper cache lock management on free

drm/pagemap_util: Ensure proper cache lock management on free

For the sake of consistency, ensure that the cache lock is always
unlocked after drm_pagemap_cache_fini. Spinlocks typically disable
preemption and if the code-path missing the unlock is hit, preemption
will remain disabled even if the lock is subsequently freed.

Fixes static analysis issue.

v2:
- Use requested code flow (Maarten)

v3:
- Clear cache->dpagemap (Matt Brost, Maarten)

v4:
- Reword commit message (Thomas)

Fixes: 77f14f2f2d73f ("drm/pagemap: Add a drm_pagemap cache and shrinker")
Signed-off-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Thomas Hellstrom <thomas.hellstrom@linux.intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patch.msgid.link/20260316151555.7553-2-jonathan.cavitt@intel.com