drm/amd/pm: Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a couple
of warnings by explicitly adding a break statement instead of letting
the code fall through to the next case, and a fallthrough pseudo-keyword
as a replacement for a /* fall through */ comment,

Notice that Clang doesn't recognize /* fall through */ comments as
implicit fall-through markings.

Link: https://github.com/KSPP/linux/issues/115
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/pm/powerplay/si_dpm.c

@@ -6200,8 +6200,8 @@ static void si_request_link_speed_change_before_state_change(struct amdgpu_devic
 		case AMDGPU_PCIE_GEN2:
 			if (amdgpu_acpi_pcie_performance_request(adev, PCIE_PERF_REQ_PECI_GEN2, false) == 0)
 				break;
+			fallthrough;
 #endif
-			/* fall through */
 		default:
 			si_pi->force_pcie_gen = si_get_current_pcie_speed(adev);
 			break;

drivers/gpu/drm/amd/pm/powerplay/smumgr/polaris10_smumgr.c

@@ -2372,6 +2372,7 @@ static int polaris10_update_smc_table(struct pp_hwmgr *hwmgr, uint32_t type)
 		break;
 	case SMU_BIF_TABLE:
 		polaris10_update_bif_smc_table(hwmgr);
+		break;
 	default:
 		break;
 	}