drm/i915/fb: drop unused but set variable cpp

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

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/ce7a5cb06c562a3399206c521a24f5091a3e7c23.1685119007.git.jani.nikula@intel.com