drm/i915: Add dpll entrypoint for dumping hw state

Remove the IS_PLATFORM() macros from intel_dump_pipe_config() and split
that logic in platform specific implementations inside the dpll code,
accessed through a platform independent interface.

v2: Rebase.
Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch> (v1)
Link: http://patchwork.freedesktop.org/patch/msgid/1483024933-3726-7-git-send-email-ander.conselvan.de.oliveira@intel.com