Exit as soon as possible if no filtering will be done.

Improves the checkasm --bench cycle count on a Snapdragon 820e:
h264_h_loop_filter_luma_8bpp_c:      72.4 ->  72.5
h264_h_loop_filter_luma_8bpp_neon:   97.1 ->  56.3
h264_v_loop_filter_luma_8bpp_c:     174.0 -> 173.5
h264_v_loop_filter_luma_8bpp_neon:   62.9 ->  60.9
h264_h_loop_filter_chroma_8bpp_c:    30.2 ->  30.3
h264_h_loop_filter_chroma_8bpp_neon: 51.6 ->  25.7
h264_v_loop_filter_chroma_8bpp_c:    57.3 ->  57.3
h264_v_loop_filter_chroma_8bpp_neon: 28.0 ->  24.0