drm/i915: Move shared dpll code to a new file

Create the new file intel_dpll_mgr.c and move the shared dpll code to
it. Follow up patches that reorganize pll handling will move more code
there and tweak the interface.

No functional changes.

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/1457451987-17466-2-git-send-email-ander.conselvan.de.oliveira@intel.com