drm/amd/display: Fix unused variable warning

[Why]
It will build failed with unused variable 'dc' with
'-Werror=unused-variable'enabled when CONFIG_DRM_AMD_DC_DCN
is not defined.
Signed-off-by: NTim Huang <tim.huang@amd.com>
Reviewed-by: NAlex Deucher <alexander.deucher@amd.com>
Reviewed-by: NHuang Rui <ray.huang@amd.com>
Reviewed-by: NAaron Liu <aaron.liu@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>

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

@@ -1027,7 +1027,6 @@ static int dm_dmub_hw_init(struct amdgpu_device *adev)
 	const unsigned char *fw_inst_const, *fw_bss_data;
 	uint32_t i, fw_inst_const_size, fw_bss_data_size;
 	bool has_hw_support;
-	struct dc *dc = adev->dm.dc;
 
 	if (!dmub_srv)
 		/* DMUB isn't supported on the ASIC. */
@@ -1123,7 +1122,7 @@ static int dm_dmub_hw_init(struct amdgpu_device *adev)
 	case IP_VERSION(3, 1, 3): /* Only for this asic hw internal rev B0 */
 		hw_params.dpia_supported = true;
 #if defined(CONFIG_DRM_AMD_DC_DCN)
-		hw_params.disable_dpia = dc->debug.dpia_debug.bits.disable_dpia;
+		hw_params.disable_dpia = adev->dm.dc->debug.dpia_debug.bits.disable_dpia;
 #endif
 		break;
 	default: