powerpc: Fix asm offsets to point to actual FP and VMX regs

The asm code assumes the FP regs are at the start of fp_state. While
this is true now, it may not always be the case and there is nothing
enforcing it.

This fixes the asm-offsets to point to the actual FP registers inside
the fp_state.  Similarly for VMX.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>