radeonsi: adjust epitch for PIPE_FORMAT_R8G8_R8B8_UNORM

radeonsi: adjust epitch for PIPE_FORMAT_R8G8_R8B8_UNORM

This fix si_compute_copy_image for yuyv image (so using PIPE_FORMAT_R8G8_R8B8_UNORM).

With this change, the following gst pipeline produce the expected results for various
image sizes (with or without AMD_DEBUG=nodma):

gst-launch-1.0 filesrc location=input.jpg ! jpegparse ! vaapijpegdec ! filesink location=output.yuv

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5841>