vp9,yuvconfig2image: set img->{w,h} to yv12->y_{width,height}

Instead of using `yv12->stride` and the aligned height including border.

This is more consistent with the setting of `yv12->border` in
`image2yuvconfig()` (`yv12->border = (yv12->y_stride - img->w) / 2`).
This also fixes a bug introduced in:
 437004c71 Seperate the border size for encoder and decoder.

Since that change, `yuvconfig2image()` was using
`VP9_ENC_BORDER_IN_PIXELS` for both encoder and decoder images. On the
decode side this led to a height that was greater than the allocated
buffer, which used `VP9_DEC_BORDER_IN_PIXELS`.

This is a backport of the same change in libaom:
 b21dac9d49 Fix a bug in image buffer conversion

Bug: 517413744
Change-Id: I37b47e4f53f6b089296c664cd3db802bd93770fb