drm/i915: Move BXT pll configuration logic to intel_dpll_mgr.c

Move the code for configurating BXT plls into the shared dpll code, so
that the platform specific details are hidden behind that 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/1457451987-17466-12-git-send-email-ander.conselvan.de.oliveira@intel.com