st/omx/enc: fix blit setup for YUV LoadImage

st/omx/enc: fix blit setup for YUV LoadImage

The blit here involves scaling since it's copying from I8 format to R8G8 format.
Half of source will be filtered out with PIPE_TEX_FILTER_NEAREST instruction, it
looks that GPU always uses the second half as source. Currently we use "1" as
the start point of x for R, then causing 1 source pixel of U component shift to
right. So "-1" should be the start point for U component.

Cc: 18.0 18.1 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>