powerpc/perf: Drop MMCRA from thread_struct

In commit 59affcd3 "Context switch more PMU related SPRs" I added more
PMU SPRs to thread_struct, later modified in commit b11ae951

. To add
insult to injury it turns out we don't need to switch MMCRA as it's
only user readable, and the value is recomputed by the PMU code.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>