drm/amd/display: remove set but not used variable 'speakers' in dce_stream_encoder.c

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

drivers/gpu/drm/amd/display/dc/dce/dce_stream_encoder.c:1339:11: warning: variable ‘speakers’ set but not used [-Wunused-but-set-variable]

It is introduced by commit 4562236b ("drm/amd/dc:
Add dc display driver (v2)"), but never used, so remove it.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zheng Bin <zhengbin13@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>