drm/kmb: Remove an unnecessary NULL check

The NULL checking isn't done consistently in this function and it leads
to a static checker warning:

    drivers/gpu/drm/kmb/kmb_drv.c:561 kmb_pm_suspend()
    error: we previously assumed 'drm' could be null (see line 559)

Fortunately "drm" cannot be NULL at this point so the check can just be
removed.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Anitha Chrisanthus <anitha.chrisanthus@intel.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201117072137.GB1111239@mwanda