drm/i915: Move pll state commit into intel_modeset_update_state

It's really part of the "push all new_* state into current state
pointers" done in that function. So let's move it there to make this
clear.

Also, with the conversion done the num_shared_dpll check the function
does in it's loop is enough, so we can drop the check for the dpll
compute callback, too.

Cc: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>