Move execlist init into a common engine setup. As it is
common to both guc and hw execlists.

v2: rebase with csb changes
v3: rebase
Signed-off-by: NMika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20170922124307.10914-2-mika.kuoppala@intel.com