drm/radeon/r300: Mark expected switch fall-throughs

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

Notice that in this particular case, I replaced "Pass through." with
"Fall through.", which is what GCC is expecting to find.

Addresses-Coverity-ID: 114734 ("Missing break in switch")
Addresses-Coverity-ID: 114735 ("Missing break in switch")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>