drm/amd: Add missing kdoc for amd_ip_funcs `complete` callback

The `complete` callback should be described in kernel doc.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Closes: https://lore.kernel.org/linux-next/20250619205931.41cf9332@canb.auug.org.au/


Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://lore.kernel.org/r/20250620041420.3585005-1-superm1@kernel.org


Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>