drm/amd/display: get tail pipe before aquire free pipe

[Why]
For some complicated blending transition cases, the head
pipe of the second stream may end up being a higher pipe
index than the free pipe.  In those cases dc_add_plane_to_context
will incorrectly set the tail_pipe to the free pipe, which
will cause the top_pipe and bottom_pipe to be the same

[How]
Move the call to resource_get_tail_pipe_for_stream() to be
before call to acquire_free_pipe_for_stream().

Signed-off-by: Eric Bernstein <eric.bernstein@amd.com>
Reviewed-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>