drm/amd/display: Fix null timing generator resource

[Why]
For some customer blending transition cases, the
available pipe for second stream is a pipe index that is
greater than the number of timing generators, which
can cause a problem in acquire_first_free_pipe since it
assumes same index for pipe and timing generator

[How]
Added logic to use last timing generator index
if the pipe index is greater than number of timing generators.

Acked-by: Alan Liu <HaoPing.Liu@amd.com>
Signed-off-by: Eric Bernstein <eric.bernstein@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>