drm/i915: Reset CSB read pointer in ring init

drm/i915: Reset CSB read pointer in ring init

A previous commit enabled execlists by default:

       commit 81a9c90e1fb8 ("drm/i915/bdw: Enable execlists by default where supported")

This allowed routine testing of execlists which exposed a regression when
resuming from suspend.  The cause was tracked down the to recent changes to the
ring init sequence:

       commit 842e694c8fbe ("drm/i915: Only init engines once")

During a suspend/resume cycle the hardware Context Status Buffer write pointer
is reset.  However since the recent changes to the init sequence the software CSB
read pointer is no longer reset.  This means that context status events are not
handled correctly and new contexts are not written to the ELSP, resulting in an
apparent GPU hang.

Pending further changes to the ring init code, just move the
ring->next_context_status_buffer initialization into gen8_init_common_ring to
fix this regression.

v2: Moved init into gen8_init_common_ring rather than context_enable after
feedback from Daniel Vetter.  Updated commit msg to reflect this and also cite
commits related to the regression.  Fixed bz link to correct bug.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=88096
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Dave Gordon <david.s.gordon@intel.com>
Signed-off-by: Thomas Daniel <thomas.daniel@intel.com>
Reviewed-by: Dave Gordon <david.s.gordon@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>