drm/amd/display: Fix fall-through warning for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix
the following warning by replacing a /* fall through */ comment
with the new pseudo-keyword macro fallthrough:

rivers/gpu/drm/amd/amdgpu/../display/dc/dce/dce_aux.c:672:4: warning: unannotated fall-through between switch labels [-Wimplicit-fallthrough]
                        case AUX_TRANSACTION_REPLY_I2C_OVER_AUX_DEFER:
                        ^

Notice that Clang doesn't recognize /* fall through */ comments as
implicit fall-through markings, so in order to globally enable
-Wimplicit-fallthrough for Clang, these comments need to be
replaced with fallthrough; in the whole codebase.

Link: https://github.com/KSPP/linux/issues/115


Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>