juno/ppu: Simplify get_ctx function

The get_ctx function would always return SUCCESS, so it can
be simplified and all checks for returned status can be
removed.
This patch makes such function plainer.

Change-Id: I031e00fa7349171c16f8c2304e3c5b2cbaeea731
Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>