drm/i915: Avoid GPU hang when coming out of s3 or s4

This patch fixes a timing issue that causes a GPU hang when the system
comes out of power saving.

During pm_resume, We are submitting batchbuffers before enabling
Interrupts this is causing us to miss the context switch interrupt,
and in consequence intel_execlists_handle_ctx_events is not triggered.

This patch is based on a patch from Deepak S <deepak.s@intel.com>
from another platform.

The patch fixes an issue introduced by:
  commit e7778be1


  drm/i915: Fix startup failure in LRC mode after recent init changes

The above patch added a call to init_context() to fix an issue introduced
by a previous patch. But, it then opened up a small timing window for the
batches being added by the init_context (basically setting up the context)
to complete before the interrupts have been turned on, thus hanging the
GPU.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=89600
Cc: stable@vger.kernel.org # 4.0+
Signed-off-by: Peter Antoine <peter.antoine@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
[Jani: fixed typo in subject, massaged the comments a bit]
Signed-off-by: Jani Nikula <jani.nikula@intel.com>