drm/i915: Store a direct pointer to shared dpll in intel_crtc_state

Change the type of intel_crtc_state->shared_dpll to be a pointer to a
shared dpll. With this there is no need to first convert the id stored
in the crtc state to a pointer in order to use it. It does introduce a
bit of hassle on doing the opposite.

The long term objective is to hide details about dpll ids behind the
shared dpll interface.

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1457451987-17466-5-git-send-email-ander.conselvan.de.oliveira@intel.com