drm/amd/display: remove set but unused variable

Fixes the following:
drivers/gpu/drm/amd/amdgpu/../display/dc/dcn32/dcn32_hwseq.c:428:33: warning: variable 'old_pipe' set but not used

Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>