drm/i915: s/update/compute/ for gmch dpll register functions

I was momentarily confused until I've double-checked that these
functions really only compute state and don't update the hardware
state. They once did that, but since Ander's rework of the dpll
computation flow that's no longer the case.

Rename them to avoid further confusion.

Note that the ilk code already follows the compute_dpll naming scheme
for computing the actual register value. DDI code goes with _calc_,
but that is close enough.

Cc: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>