drm/amdgpu/display: Protect some functions with CONFIG_DRM_AMD_DC_DCN

Protect remove_hpo_dp_link_enc_from_ctx() and release_hpo_dp_link_enc()
with CONFIG_DRM_AMD_DC_DCN as the functions are only called from code
that is protected by CONFIG_DRM_AMD_DC_DCN.  Fixes build fail with
-Werror=unused-function.

Fixes: 9b0e0d433f74 ("drm/amd/display: Add dependant changes for DCN32/321")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: Aurabindo Pillai <aurabindo.pillai@amd.com>