drm/i915: Create context desc template when context is created

Move the invariant parts of context desc setup from execlist init
to context creation. This is advantageous when we need to
create different templates based on the context parametrization,
ie. for svm capable contexts.

v2: s/create/default, remove engine->ctx_desc_template

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1485522189-31984-1-git-send-email-mika.kuoppala@intel.com