KVM: arm64: Fix debug register indexing

Commit 03fdfb26 ("KVM: arm64: Don't write junk to sysregs on
reset") flipped the register number to 0 for all the debug registers
in the sysreg table, hereby indicating that these registers live
in a separate shadow structure.

However, the author of this patch failed to realise that all the
accessors are using that particular index instead of the register
encoding, resulting in all the registers hitting index 0. Not quite
a valid implementation of the architecture...

Address the issue by fixing all the accessors to use the CRm field
of the encoding, which contains the debug register index.

Fixes: 03fdfb26 ("KVM: arm64: Don't write junk to sysregs on reset")
Reported-by: Ricardo Koller <ricarkol@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Cc: stable@vger.kernel.org