radv: fix a GPU hang when MRTs are sparse

radv: fix a GPU hang when MRTs are sparse

When the i-th target format is set, all previous target formats
must be non-zero to avoid hangs. In other words, without this
if a fragment shader exports mrt0, mrt2 and mrt3, the GPU hangs
because the target format of mrt1 is zero.

This fixes DXVK GPU hangs with "Seven: The Days Long Gone",
"GTA V" and probably more games.

Cc: "18.0" 18.1" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>