drm/amd/display: Fix encoder disable logic

[WHY]
DENTIST hangs when OTG is off and encoder is on. We were not
disabling the encoder properly when switching from extended mode to
external monitor only.

[HOW]
Disable the encoder using an existing enable/disable fifo helper instead
of enc35_stream_encoder_enable.

Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Nicholas Susanto <nicholas.susanto@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>