drm/amdgpu/display: fix dal_allocation documentation

Add missing structure elements.

Fixes: 0dd79532 ("drm/amdgpu/display: Implement functions to let DC allocate GPU memory")
Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>