pft: fix pft_allow_merge_bio() to check if key is loaded

Encrypted BIOs are created only when file encryption is enabled,
which happens only when key is loaded.
If key is not loaded then non-encrypted BIOs can safely merge.
This fix makes PFT hook silent before activation
and fix Factory-Data-Reset (FDR) issue.

Change-Id: I7763ee540f157fe590f400467dbf73b62adcf407
Signed-off-by: Amir Samuelov <amirs@codeaurora.org>