vp8e_get_preview fixed for resized frames

preview_img d_w and d_h along with w and h
would not be updated for resized frames.

now uses sd.y_width and sd.y_height

Change-Id: I52241de4cc1de5e73f865e668bd70a7cbd954390
diff --git a/vp8/vp8_cx_iface.c b/vp8/vp8_cx_iface.c
index b23bd95..ea99c61 100644
--- a/vp8/vp8_cx_iface.c
+++ b/vp8/vp8_cx_iface.c
@@ -912,8 +912,8 @@
         ctx->preview_img.x_chroma_shift = 1;
         ctx->preview_img.y_chroma_shift = 1;
 
-        ctx->preview_img.d_w = ctx->cfg.g_w;
-        ctx->preview_img.d_h = ctx->cfg.g_h;
+        ctx->preview_img.d_w = sd.y_width;
+        ctx->preview_img.d_h = sd.y_height;
         ctx->preview_img.stride[VPX_PLANE_Y] = sd.y_stride;
         ctx->preview_img.stride[VPX_PLANE_U] = sd.uv_stride;
         ctx->preview_img.stride[VPX_PLANE_V] = sd.uv_stride;