drm/amd/display: Remove set but not used variables 'pp_smu', 'old_pipe'

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

drivers/gpu/drm/amd/display/dc/dce110/dce110_hw_sequencer.c: In function dce110_enable_audio_stream:
drivers/gpu/drm/amd/display/dc/dce110/dce110_hw_sequencer.c:949:23: warning: variable pp_smu set but not used [-Wunused-but-set-variable]
drivers/gpu/drm/amd/display/dc/dce110/dce110_hw_sequencer.c: In function dce110_disable_audio_stream:
drivers/gpu/drm/amd/display/dc/dce110/dce110_hw_sequencer.c:983:23: warning: variable pp_smu set but not used [-Wunused-but-set-variable]
drivers/gpu/drm/amd/display/dc/dce110/dce110_hw_sequencer.c: In function dce110_program_front_end_for_pipe:
drivers/gpu/drm/amd/display/dc/dce110/dce110_hw_sequencer.c:2429:19: warning: variable old_pipe set but not used [-Wunused-but-set-variable]

'pp_smu' is not used since commit 170a2398 ("drm/amd/display:
make clk_mgr call enable_pme_wa")

'old_pipe' is not used since commit 65d38262 ("drm/amd/display:
fbc state could not reach while enable fbc")

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>