gallium/vl: don't set PIPE_HANDLE_USAGE_EXPLICIT_FLUSH

because vl doesn't call flush_resource and I wasn't able to find
all places where flush_resource needs to be called.

This fixes corrupted / unflushed surfaces with fullscreen videos on Raven.

Cc: 19.1 19.2 <mesa-stable@lists.freedesktop.org>
(cherry picked from commit f52afdf6)