drm/i915: Move shared dpll function prototypes to intel_dpll_mgr.h

Move shared dpll function prototype together with other shared dpll
definitions.

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/1457451987-17466-7-git-send-email-ander.conselvan.de.oliveira@intel.com