st/mesa: Set pipe_image_view::shader_access in PBO readpixels.

st/mesa: Set pipe_image_view::shader_access in PBO readpixels.

Commit 8b626a22b24089edf90cb1b06e5b1895bb36c61b introduced a new
pipe_image_view::shader_access field, indicating the access mode
specified in the shader.  st/mesa's built-in PBO download shader
creates a write-only image buffer, so we should flag it as such.

Nobody uses this field yet (Iris will), so we don't need to backport
this fix to stable branches.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>