powerpc/perf: Drop MMCRA from thread_struct

In commit 59affcd "Context switch more PMU related SPRs" I added more
PMU SPRs to thread_struct, later modified in commit b11ae95. 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>