drm/amd/display: Refactor amdgpu_dm_initialize_default_pipeline

drm/amd/display: Refactor amdgpu_dm_initialize_default_pipeline

[Why & How]
Extract amdgpu_dm_initialize_default_pipeline() into a new
STATIC_IFN_KUNIT helper amdgpu_dm_build_default_pipeline().

This separation makes the pipeline-building logic testable via
KUnit without pulling in amdgpu_device and its dependencies
that are unavailable in the UML KUnit build environment.

Assisted-by: Copilot:Claude-Sonnet-4.6
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Ray Wu <ray.wu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>