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: d38ceaf9 ("drm/amdgpu: add core driver (v4)")
Signed-off-by: yu kuai <yukuai3@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>