drm/i915: annotate maybe unused but set intel_plane_state variables

Prepare for re-enabling -Wunused-but-set-variable.

for_each_new_intel_plane_in_state() requires passing in a struct
intel_plane_state pointer, which it uses, but in many places this leads
to warning about unused but set variables. Annotate them with
__maybe_unused.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/438ff3b257b7f85ecca5750ae8687336faee0a79.1685119007.git.jani.nikula@intel.com