drm/i915: Inline engine->init_context into its caller

We only use the init_context vfunc once while recording the default
context state, and we use the same sequence in each backend (eliding
steps that do not apply). Remove the vfunc for simplicity and
de-duplication.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190729113720.24830-1-chris@chris-wilson.co.uk