egl: stop claiming support for pbuffer + msaa

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.

I also found that some Qt example did a workaround because of this
crash: https://bugreports.qt.io/browse/QTBUG-47509

v2: Ian pointed out that v1 removed support for all multisample
    configs, including window ones. This one removes pbuffer bit
    when adding configs, now only pbuffer+msaa gets rejected and
    window+msaa continues to work. Fixed also comment (Emil)

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>