drm/i915: Refactor platform specifics out of intel_get_shared_dpll()

The function intel_get_shared_dpll() had a more or less generic
implementation with some platform specific checks to handle smaller
differences between platforms. However, the minimalist approach forces
bigger differences between platforms to be implemented outside of the
shared dpll code (see the *_ddi_pll_select() functions in intel_ddi.c,
for instance).

This patch changes the implementation of intel_get_share_dpll() so that
a completely platform specific version can be used, providing helpers to
reduce code duplication. This should allow the code from the ddi pll
select functions to be moved, and also make room for making more dplls
managed by the shared dpll infrastructure.

v2: WARN_ON(!dpll_mgr) in intel_get_shared_dpll(). (Maarten)

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/1457451987-17466-9-git-send-email-ander.conselvan.de.oliveira@intel.com