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: e6285ea5 ("egl: Replace the robustness DRI2 ext check with a pipe cap query.") Closes: #8066 (closed)