drm/i915/display: refactor out initial plane config for crtcs

This just pulls this out into a function so it can be moved to
another file easier.

Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211012043502.1377715-4-airlied@gmail.com