st/mesa: account for "loose", per-mipmap level textures in CopyImageSubData

st/mesa: account for "loose", per-mipmap level textures in CopyImageSubData

We may have "loose", per-image gallium resources. The src_image->Level
may not match the gallium resource texture level. In such case it is
prescribed (in st_AllocTextureImageBuffer) to specify mipmap level
as zero.

Fixes: f04f13622f3e71bee057d60a6be9c53b92b56cc9
Signed-off-by: Danylo Piliaiev <danylo.piliaiev@globallogic.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5477>