drm/v3d: nullify pointer se with a NULL

Currently a plain integer is being used to nullify the pointer
struct v3d_submit_ext *se. Use NULL instead. Cleans up sparse
warnings:
drivers/gpu/drm/v3d/v3d_gem.c:777:53: warning: Using plain integer as
NULL pointer
drivers/gpu/drm/v3d/v3d_gem.c:1010:45: warning: Using plain integer as
NULL pointer

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Reviewed-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Melissa Wen <melissa.srw@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1634282081-72255-1-git-send-email-yang.lee@linux.alibaba.com