tpm: migrate tpm2_get_tpm_pt() to use struct tpm_buf

In order to make struct tpm_buf the first class object for constructing TPM
commands, migrate tpm2_get_tpm_pt() to use it.

Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Reviewed-by: Nayna Jain <nayna@linux.vnet.ibm.com>
Tested-by: Nayna Jain <nayna@linux.vnet.ibm.com>
Reviewed-by: Jerry Snitselaar <jsnitsel@redhat.com>