drm/msm: use %z format modifier for printing size_t

The return type of ARRAY_SIZE() is size_t, so we have to use
%zu instead of %lu to avoid this warning:

drivers/gpu/drm/msm/msm_gpu.c: In function 'msm_gpu_init':
drivers/gpu/drm/msm/msm_gpu.c:742:31: error: format '%lu' expects argument of type 'long unsigned int', but argument 7 has type 'unsigned int' [-Werror=format=]

The warning it otherwise harmless as size_t is always the
same size as unsigned long in all supported architectures,
but gcc doesn't know that.

Fixes: c2fceabca6d5 ("drm/msm: Support multiple ringbuffers")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Rob Clark <robdclark@gmail.com>