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>