Don't zero uv width/height when forcing mono input

Scale back the last commit 972797c6ad3f704ba222ab3126e02713617d8ddf.

Don't set uv width/height to 0 when forcing monochrome input, because I
did not have time to analyze if any code would be broken by a zero uv
width or height. Also leave subsampling_x and subsampling_y unchanged
for consistency. (We could set subsampling_x and subsampling_y to 1 and
recalculate uv width and height, but I don't want to do unneeded work.)

BUG=aomedia:2722

Change-Id: I77b1f492041756093978b8fb152bba7bda5d21af
diff --git a/av1/av1_cx_iface.c b/av1/av1_cx_iface.c
index 294ce92..4707896 100644
--- a/av1/av1_cx_iface.c
+++ b/av1/av1_cx_iface.c
@@ -2161,8 +2161,6 @@
       if (ctx->cfg.monochrome) {
         sd.u_buffer = sd.v_buffer = NULL;
         sd.uv_stride = 0;
-        sd.uv_width = sd.uv_height = sd.uv_crop_width = sd.uv_crop_height = 0;
-        sd.subsampling_x = sd.subsampling_y = 1;
         sd.monochrome = 1;
       }
       use_highbitdepth = (sd.flags & YV12_FLAG_HIGHBITDEPTH) != 0;