drm/i915: Extract ring init from hw_init

The ring initialization will differ a bit in upcoming generations, and
this split will prepare the code for what's needed.

This patch also fixes a bug introduced in:
commit 99433931
Author: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Date:   Tue Jan 22 14:12:17 2013 +0200

    drm/i915: use gem_set_seqno() on hardware init

After doing the extraction, the bad error handling became obvious.  I
acknowledge that this should be two patches, but it's a pretty
small/trivial patch. If requested, I can certainly do the fix as a
distinct patch.

v2: Should be cleanup blt, not init blt on failure (Chris)

v3: Forgot to git add on v2

Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>