drm/i915: annotate maybe unused but set intel_crtc_state variables

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

for_each_new_intel_crtc_in_state() requires passing in a struct
intel_crtc_state pointer, which it uses, but in a few 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/eb041f426bc3d76ef7a0ea906f99367cbf439b1a.1685119007.git.jani.nikula@intel.com