drm/amd/display: remove set but not used variable 'dc'

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

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c:8665:13: warning: variable ‘dc’ set but not used [-Wunused-but-set-variable]

It is not used since commit d1ebfdd8 ("drm/amd/display:
Unify psr feature flags")

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