cuda: factor out timebase-derived counter value and load time

Commit b981289c "PPC: Cuda: Use cuda timer to expose tbfreq to guest" altered
the timer calculations from those based upon the hardware CUDA clock frequency
to those based upon the CPU timebase frequency.

In fact we can isolate the differences to 2 simple changes: one to the counter
read value and another to the counter load time. Move these changes into
separate functions so the implementation can be swapped later.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>