accel/amdxdna: Use rcu_access_pointer for __rcu pointer

accel/amdxdna: Use rcu_access_pointer for __rcu pointer

Use rcu_access_pointer for pid in struct drm_file. This fixes sparse
warning.

Fixes: be462c97b7df ("accel/amdxdna: Add hardware context")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202412311210.LfeHTzLw-lkp@intel.com/
Signed-off-by: Lizhi Hou <lizhi.hou@amd.com>
Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250102212244.1586340-4-lizhi.hou@amd.com