egl: Replace the robustness DRI2 ext check with a pipe cap query.

egl: Replace the robustness DRI2 ext check with a pipe cap query.

This means that swkms will now report it with llvmpipe.  drisw reported it
with llvmpipe, and it appears to have been an oversight that got
refactored into an obvious "if !swkms" check later.

Reviewed-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20171>