radeonsi: disable SDMA on gfx8 to fix corruption on RX 580

Closes: #1399
Closes: #1889

Cc: 19.2 19.3 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Reviewed-By: Timur Kristóf <timur.kristof@gmail.com>
(cherry picked from commit e5167a92)

Conflicts:
	src/gallium/drivers/radeonsi/si_pipe.c