drm/xe/userptr: Drop bogus static from finish in force_invalidate

drm/xe/userptr: Drop bogus static from finish in force_invalidate

The local "finish" pointer in xe_vma_userptr_force_invalidate() is
unconditionally written before each read, so the static storage class
serves no purpose. Worse, it makes the variable a process-wide shared
slot: the function's per-VM asserts do not exclude concurrent callers
on different VMs, so two such callers can race on the slot and take
the wrong if (finish) branch.

The function is gated by CONFIG_DRM_XE_USERPTR_INVAL_INJECT
(developer/test option, default n), so production builds are
unaffected.

Drop the static.

Fixes: 18c4e536959e ("drm/xe/userptr: Convert invalidation to two-pass MMU notifier")
Assisted-by: Claude:claude-opus-4.7
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Zongyao Bai <zongyao.bai@intel.com>
Link: https://patch.msgid.link/20260625224452.3243231-1-shuicheng.lin@intel.com
Signed-off-by: Shuicheng Lin <shuicheng.lin@intel.com>
(cherry picked from commit ed382e3b07fae51a09d7290485bff0592f6b168b)
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>