drm/xe: Use drmm_mutex_init for VRAM manager lock

drm/xe: Use drmm_mutex_init for VRAM manager lock

Replace mutex_init()/mutex_destroy() with drmm_mutex_init() for the
VRAM manager lock. This leverages DRM managed infrastructure to
automatically destroy the mutex during device teardown, eliminating
manual mutex_destroy() calls in both the normal fini path and the
gpu_buddy_init() error path.

Assisted-by: Claude:claude-opus-4.6
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patch.msgid.link/20260429093138.3899280-2-tejas.upadhyay@intel.com
Signed-off-by: Tejas Upadhyay <tejas.upadhyay@intel.com>