drm/i915: Move HSW/BDW pll selection logic to intel_dpll_mgr.c

Move the code for selecting and configuring HSW/BDW DDI PLLs into the
shared dpll infrastructure. With this most of the PLL selection logic
for those platforms is in one place. DisplayPort is handled separately,
but that should be fixed on a follow up patch. It also allows a small
clean up of the SPLL logic.

v2: Rebase.

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/1457451987-17466-10-git-send-email-ander.conselvan.de.oliveira@intel.com