drm/i915: move intel_display.c function declarations

Move the declarations of functions in intel_display.c to
intel_display.h. There is still plenty of cleanup to do in
intel_display.[ch], but one step at a time.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/ebe19d3c508faa82e651ab3c8eb7a0975bfa52b7.1565085691.git.jani.nikula@intel.com