drm/i915: Move shared dpll struct definitions to separate header file

Move the declarations related to shared dplls from i915_drv.h to their
own header file.

The code that became the shared dpll infrastructre was first introcude
in commit ee7b9f93 ("drm/i915: manage PCH PLLs separately from
pipes"), hence the 2012-2016 copyright years in the new header file.

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/1457451987-17466-6-git-send-email-ander.conselvan.de.oliveira@intel.com