drm/i915/gem: annotate maybe unused but set variable c

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

The variable 'c' appears unused, but I'm not sure what should be done
with it. Annotate it with __maybe_unused.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/9ee9e7d7a0a7ad4ff03c14e64b95d3fbcb7885a4.1685119007.git.jani.nikula@intel.com