drm/amd/display: Disconnect mpcc when changing tg

[Why]
This fixes an mpc programming error for the following sequence of
atomic commits when pipe split is enabled:

Commit 1: CRTC0 (plane 4, plane 3)

Pipe 0: old_plane_state = A0, new_plane_state = A1,   new_tg = T0
Pipe 1: old_plane_state = B0, new_plane_state = B1,   new_tg = T0
Pipe 2: old_plane_state = A0, new_plane_state = A1,   new_tg = T0
Pipe 3: old_plane_state = B0, new_plane_state = B1,   new_tg = T0

Commit 2: CRTC0 (plane 3), CRTC1 (plane 2)

Pipe 0: old_plane_state = A1, new_plane_state = A2,   new_tg = T0
Pipe 1: old_plane_state = B1, new_plane_state = B2,   new_tg = T1
Pipe 2: old_plane_state = A1, new_plane_state = NULL, new_tg = NULL
Pipe 3: old_plane_state = B1, new_plane_state = NULL, new_tg = NULL

In the second commit the assertion for mpcc in use is hit because
mpcc disconnect never occurs for pipe 1. This is because the stream
changes for pipe 1 and the opp_list is empty.

This sequence occurs when running the
"igt@kms_plane_multiple@atomic-pipe-A-tiling-none" test with two
displays connected.

[How]
Expand the reset condition to include:

"old_pipe_ctx->stream_res.tg != new_pipe_ctx->stream_res.tg"

...but only when the plane state is non-NULL for both old and new.

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>