drm/i915: Split intel_get_shared_dpll() into smaller functions

Make the code neater by splitting the code for platforms with fixed PLL
to their own functions and splitting the logic for finding a shareable
or unused pll from the logic for setting it up.

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/1457451987-17466-4-git-send-email-ander.conselvan.de.oliveira@intel.com