drm/i915: Rename plane_config to initial_plane_config

This vfunc and related structure are only used for fast boot, so let's
rename them to not take them as general purpose ones.

v2: Fix conflicts caused by the introduction of struct intel_crtc_state

Reviewed-By: Tvrtko Ursulin <tvrtko.ursulin@intel.com> (v1)
Suggested-by: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>