egl/drm: reinstate (kms_)swrast support

egl/drm: reinstate (kms_)swrast support

With earlier commit we've added a generic LIBGL_ALWAYS_SOFTWARE handling
yet did not consider that the existing codebase unconditionally errors
out when set. That was fixed with a latter commit, while the fix itself
added erroneous restriction for egl/drm.

As mentioned in the report - the feature was working for ages. It was a
Gnome developer who added kms_swrast support for gbm in the first place.

Admittedly kms_swrast is somewhat in the middle between traditional
swrast and HW drivers, regardless - reinstate support.

Fixes: 47273d7312c ("egl: set UseFallback if LIBGL_ALWAYS_SOFTWARE is set")
Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/165
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Acked-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4084>