drm/amd/display: Set full update flag in dcn_validate_bandwidth

Doing bandwidth validation implies that this is a full update. Set the
flag inside the function in case whatever is calling
dcn_validate_bandwidth doesn't set it.

Signed-off-by: Andrew Jiang <Andrew.Jiang@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>