drm/amd/display: correct num_dsc based on HW cap

[why]
num_dsc is 3 for dcn314 based on HW capablity.

Reviewed-by: Martin Leung <Martin.Leung@amd.com>
Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Charlene Liu <Charlene.Liu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>