drm/amd/display: apply edge-case DISPCLK WDIVIDER changes to master OTG pipes only

[Why]
The edge-case DISPCLK WDIVIDER changes call stream_enc functions.
But with MPC pipes, downstream pipes have null stream_enc and will
 cause crash.

[How]
Only call stream_enc functions for pipes that are OTG master.

Reviewed-by: Alvin Lee <alvin.lee2@amd.com>
Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Samson Tam <samson.tam@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>