drm/amd/display: Add callback to set dig mode

[Why&How]
Add a missing callback to set DIG FIFO output pixel mode. This is used
when ODM combine is activated.

Fixes: 235c6763 ("drm/amd/display: add DCN32/321 specific files for Display Core")
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>