drm/amdgpu: remove set but not used variable 'dig'

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

drivers/gpu/drm/amd/amdgpu/atombios_dp.c: In function
‘amdgpu_atombios_dp_link_train’:
drivers/gpu/drm/amd/amdgpu/atombios_dp.c:716:34: warning: variable ‘dig’
set but not used [-Wunused-but-set-variable]

Fixes: d38ceaf99ed0 ("drm/amdgpu: add core driver (v4)")
Signed-off-by: yu kuai <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/amd/amdgpu/atombios_dp.c

···
 	struct drm_device *dev = encoder->dev;
 	struct amdgpu_device *adev = dev->dev_private;
 	struct amdgpu_encoder *amdgpu_encoder = to_amdgpu_encoder(encoder);
-	struct amdgpu_encoder_atom_dig *dig;
 	struct amdgpu_connector *amdgpu_connector;
 	struct amdgpu_connector_atom_dig *dig_connector;
 	struct amdgpu_atombios_dp_link_train_info dp_info;
···
 
 	if (!amdgpu_encoder->enc_priv)
 		return;
-	dig = amdgpu_encoder->enc_priv;
 
 	amdgpu_connector = to_amdgpu_connector(connector);
 	if (!amdgpu_connector->con_priv)