zink: only add srgb mutable for images with modifiers

these are supposed to be for dmabuf handling, so checking for mutable
swapchain is both pointless and wrong

Fixes: 28ee911ad67 ("zink: handle mutable swapchain images with dmabuf")

Reviewed-by: Emma Anholt <emma@anholt.net>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18358>