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

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

BugLink: http://bugs.launchpad.net/bugs/1578798
commit 5d5b7803c49bbb01bdf4c6e95e8314d0515b9484 upstream.

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>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>