powerpc: Don't disable kernel FP/VMX/VSX MSR bits on context switch

Writing the MSR is slow, so we want to avoid it whenever possible.

A subsequent patch will add a debug option that strictly manages the
FP/VMX/VSX unavailable bits. For now just remove it, matching what
we do in other areas of the kernel (eg enable_kernel_altivec()).

A context switch microbenchmark using yield():

http://ozlabs.org/~anton/junkcode/context_switch2.c

./context_switch2 --test=yield --fp 0 0

shows an improvement of almost 3% on POWER8.

Signed-off-by: Anton Blanchard <[email protected]>
Signed-off-by: Michael Ellerman <[email protected]>

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