drm/i915: Use "%zu" to format size_t

Switch to %zu for printing size_t which will
fix compilation warning for 32-bit build.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221230183500.20553-1-nirmoy.das@intel.com