drm/i915: set initialised only when init_context callback is NULL

During execlist_context_deferred_alloc() we presumed that the context is
uninitialised (we only just allocated the state object for it!) and
chose to optimise away the later call to engine->init_context() if
engine->init_context were NULL. This breaks with GVT's contexts that are
marked as pre-initialised to avoid us annoyingly calling
engine->init_context(). The fix is to not override ce->initialised if it
is already true.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chuanxiao Dong <chuanxiao.dong@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1494497262-24855-1-git-send-email-chuanxiao.dong@intel.com


Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>