drm/i915/display: refactor initial plane config to a separate file

This moves this functionality out of intel_display.c to separate
self-contained file.

Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211012043502.1377715-5-airlied@gmail.com