etnaviv: fix comment for source of etna_mesa_debug

The file and the environment variables have different names than
described in the comment. Fix the comment to avoid confusion.

Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6661>
1 file changed