dri2: Fix exposing robustness with swkms.

dri2: Fix exposing robustness with swkms.

In the original change I noticed that missing robustness on swkms seemed
to be an oversight, since it was enabled on sw-non-kms, so I exposed the
ext based on the underlying pipe query.  However it turns out that there
is a dri_screen flag for allowing robust contexts that exists to do error
checking for GLX, which was under an !swkms check.  So we would expose the
ext, but then throw an error if you tried to create one.

Fixes: e6285ea55fb2 ("egl: Replace the robustness DRI2 ext check with a pipe cap query.")
Closes: #8066
Reviewed-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20679>