Revert "drm/amdgpu/display: Protect some functions with CONFIG_DRM_AMD_DC_DCN"

This reverts commit d8e4fb91.

This is no longer necessary as newer patches require these functions
without CONFIG_DRM_AMD_DC_DCN.

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