drm/amd/display: Initialize variable with default value

Set a default value for target_div.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>