etnaviv: Hide MSAA support behind debug flag

etnaviv: Hide MSAA support behind debug flag

With the current level of MSAA support we have some real world rendering
problems in Chromium/Skia (issue #7678). Convert the MSAA support to an
explicit opt-in by settings ETNA_MESA_DEBUG=msaa.

Cc: 22.3 mesa-stable
Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19870>
(cherry picked from commit 4f5e59d1146a6d14f57d4f215a08a149bb8bf40c)