drm/i915: Do a fuller init after reset

I had this lying around from he original PPGTT series, and thought we
might try to get it in by itself.

It's convenient to just call i915_gem_init_hw at reset because we'll be
adding new things to that function, and having just one function to call
instead of reimplementing it in two places is nice.

In order to accommodate we cleanup ringbuffers in order to bring them
back up cleanly. Optionally, we could also teardown/re initialize the
default context but this was causing some problems on reset which I
wasn't able to fully debug, and is unnecessary with the previous context
init/enable split.

This essentially reverts:
commit 8e88a2bd
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Tue Jun 19 18:40:00 2012 +0200

    drm/i915: don't call modeset_init_hw in i915_reset

It seems to work for me on ILK now. Perhaps it's due to:
commit 8a5c2ae7


Author: Jesse Barnes <jbarnes@virtuousgeek.org>
Date:   Thu Mar 28 13:57:19 2013 -0700

    drm/i915: fix ILK GPU reset for render

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>