drm/amd/display: rename acquire_idle_pipe_for_layer to acquire_free_pipe_as_sec_dpp_pipe

[why]
Secondary DPP pipes are used for rendering secondary layers of planes.
The name "for layer" doesn't make it obvious. The function is acquiring
a free pipe as secondary dpp pipe only. We rename it so it is more obvious.
In a future follow up change, we want to add functions to acquire free pipe as
opp head pipe or otg master pipe as well. They will have their separate
allocation priority.

Reviewed-by: Jun Lei <jun.lei@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>