Revert "drm/amd/display: Skip entire amdgpu_dm build if !CONFIG_DRM_AMD_DC"

This reverts commit 1b097bcd.

[WHY & HOW]
The writeback series cause a regression in thunderbolt display.

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