msm/fbdev: fix unused variable warning with clang.

clang builds showed this:
drivers/gpu/drm/msm/msm_fbdev.c:144:6: error: variable 'helper' is used uninitialized whenever 'if' condition is true [-Werror,-Wsometimes-uninitialized]
        if (!fbdev)
            ^~~~~~

Fixes: 3fb1f62f ("drm/fb-helper: Remove drm_fb_helper_unprepare() from drm_fb_helper_fini()")
Signed-off-by: Dave Airlie <airlied@redhat.com>