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

[WHY]
Previously this only excluded build for a few amdgpu_dm
binaries which makes no sense.

[HOW]
Wrap the entire Makefile in "ifneq ($(CONFIG_DRM_AMD_DC),)"

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