drm/amd/display: remove set but not used variable 'old_plane_crtc'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c: In function dm_determine_update_type_for_commit:
drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c:6516:36: warning: variable old_plane_crtc set but not used [-Wunused-but-set-variable]

It is introduced by commit a87fa993 ("drm/amd/display:
Build stream update and plane updates in dm"), but never used,
so remove it.
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: zhengbin <zhengbin13@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -7611,7 +7611,7 @@ dm_determine_update_type_for_commit(struct amdgpu_display_manager *dm,
 	int i, j, num_plane, ret = 0;
 	struct drm_plane_state *old_plane_state, *new_plane_state;
 	struct dm_plane_state *new_dm_plane_state, *old_dm_plane_state;
-	struct drm_crtc *new_plane_crtc, *old_plane_crtc;
+	struct drm_crtc *new_plane_crtc;
 	struct drm_plane *plane;
 
 	struct drm_crtc *crtc;
@@ -7657,7 +7657,6 @@ dm_determine_update_type_for_commit(struct amdgpu_display_manager *dm,
 			uint64_t tiling_flags;
 
 			new_plane_crtc = new_plane_state->crtc;
-			old_plane_crtc = old_plane_state->crtc;
 			new_dm_plane_state = to_dm_plane_state(new_plane_state);
 			old_dm_plane_state = to_dm_plane_state(old_plane_state);