drm/msm: fix %s null argument error

drm/msm: fix %s null argument error

The following build error was triggered because of NULL string argument:

BUILDSTDERR: drivers/gpu/drm/msm/disp/mdp5/mdp5_smp.c: In function 'mdp5_smp_dump':
BUILDSTDERR: drivers/gpu/drm/msm/disp/mdp5/mdp5_smp.c:352:51: error: '%s' directive argument is null [-Werror=format-overflow=]
BUILDSTDERR:   352 |                         drm_printf(p, "%s:%d\t%d\t%s\n",
BUILDSTDERR:       |                                                   ^~
BUILDSTDERR: drivers/gpu/drm/msm/disp/mdp5/mdp5_smp.c:352:51: error: '%s' directive argument is null [-Werror=format-overflow=]

This happens from the commit a61ddb4393ad ("drm: enable (most) W=1
warnings by default across the subsystem"). Using "(null)" instead
to fix it.

Fixes: bc5289eed481 ("drm/msm/mdp5: add debugfs to show smp block status")
Signed-off-by: Sherry Yang <sherry.yang@oracle.com>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/611071/
Link: https://lore.kernel.org/r/20240827165337.1075904-1-sherry.yang@oracle.com
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>