egl/dri2: Stop disabling pbuffer support on msaa configs

egl/dri2: Stop disabling pbuffer support on msaa configs

We started doing this in:

    commit 4d6d55deef291b489af4d7870c6f5eb223c8da5d
    Author: Tapani Pälli <tapani.palli@intel.com>
    Date:   Mon Sep 26 10:03:32 2016 +0300

        egl: stop claiming support for pbuffer + msaa

        This fixes a crash in egl-create-msaa-pbuffer-surface Piglit test
        and same crash in many dEQP EGL tests.

Whatever bug that was papering over appears to be fixed by now, I can
no longer reproduce that crash with piglit. Furthermore, disabling that
bit in the generic dri2 code had the side effect that the surfaceless
platform would advertise EGLConfigs with _no_ supported surface types
(since surfaceless only supports pbuffers).

Reviewed-by: Emma Anholt <emma@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/1859>