drm/amd/display: Remove needless cast in amdgpu_dm_connector_init()

The cast of dc_link is redundant.

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Acked-by: Andrey Grodzovsky <andey.grodzovsky@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>