Perf: arm64: Use proper API to get task cpu

Instead of directly accessing thread_info structure to get the
task cpu use the appropriate wrapper API. This is needed for
subsequent patches refactors the location of task cpu.

Change-Id: Ib87f42481beee04ba646aee3b0b4b5a43862713e
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>