drm/amdgpu/si_dpm: Mark expected switch fall-throughs

In preparation to enabling -Wimplicit-fallthrough, mark switch
cases where we are expecting to fall through.

Warning level 3 was used: -Wimplicit-fallthrough=3

This patch is part of the ongoing efforts to enable
-Wimplicit-fallthrough.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/si_dpm.c

@@ -6216,10 +6216,12 @@ static void si_request_link_speed_change_before_state_change(struct amdgpu_devic
 			si_pi->force_pcie_gen = AMDGPU_PCIE_GEN2;
 			if (current_link_speed == AMDGPU_PCIE_GEN2)
 				break;
+			/* fall through */
 		case AMDGPU_PCIE_GEN2:
 			if (amdgpu_acpi_pcie_performance_request(adev, PCIE_PERF_REQ_PECI_GEN2, false) == 0)
 				break;
 #endif
+			/* fall through */
 		default:
 			si_pi->force_pcie_gen = si_get_current_pcie_speed(adev);
 			break;