drm/radeon/radeon_dp_mst: Remove unused variable 'ret' from radeon_mst_encoder_dpms()

drm/radeon/radeon_dp_mst: Remove unused variable 'ret' from radeon_mst_encoder_dpms()

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/radeon/radeon_dp_mst.c: In function ‘radeon_mst_encoder_dpms’:
 drivers/gpu/drm/radeon/radeon_dp_mst.c:366:6: warning: variable ‘ret’ set but not used [-Wunused-but-set-variable]

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>