Fixes gcc '-Wunused-but-set-variable' warning:

drivers/gpu/drm/drm_plane.c: In function 'drm_mode_getplane_res':
drivers/gpu/drm/drm_plane.c:475:26: warning:
 variable 'config' set but not used [-Wunused-but-set-variable]
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1536646814-186429-1-git-send-email-yuehaibing@huawei.com