DPE: Restructure derive_context_request code

Since the function code is already too long and to
facilitate futher update, split it into
sub-functions.

Signed-off-by: Maulik Patel <maulik.patel@arm.com>
Change-Id: Id3801cc81f03d8860e78e467406c083e87c12e8b
4 files changed