drm/radeon: fix vertical bars appear on monitor (v2)

When crtc/timing is disabled on boot the dig block
should be stopped in order ignore timing from crtc,
reset the steering fifo otherwise we get display
corruption or hung in dp sst mode.

v2: agd: fix coding style

Signed-off-by: Vitaly Prosyak <vitaly.prosyak@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>