arm64/vmm: Fix handling of MDCR_EL2.TDE

TDE (make EL2 the target EL for debug exceptions) is set both when
setting guest breakpoints and when single-stepping the guest. In some
cases we may configure both capabilities, and when subsequently
disabling one of them we need to take care to avoid clearing TDE if the
other is still configured.

MFC after: 3 days
Fixes: 75cb949228bb ("arm64/vmm: Add breakpoint and single-stepping support")
Sponsored by: CHERI Research Centre (EPSRC grant UKRI3001)

(cherry picked from commit b6f25aca11c98acc49f111f2899ecd4e96c7debd)