tpm: consolidate the TPM startup code

Consolidated all the "manual" TPM startup code to a single function
in order to make code flows a bit cleaner and migrate to tpm_buf.

Tested-by: Stefan Berger <stefanb@linux.vnet.ibm.com>
Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Signed-off-by: James Morris <james.l.morris@oracle.com>