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>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -8662,14 +8662,12 @@ static bool amdgpu_dm_link_setup_psr(struct dc_stream_state *stream)
 	struct dc_link *link = NULL;
 	struct psr_config psr_config = {0};
 	struct psr_context psr_context = {0};
-	struct dc *dc = NULL;
 	bool ret = false;
 
 	if (stream == NULL)
 		return false;
 
 	link = stream->link;
-	dc = link->ctx->dc;
 
 	psr_config.psr_version = link->dpcd_caps.psr_caps.psr_version;