drm/i915: do not export the instances of struct intel_ring_buffer

Introduce intel_init_render_ring_buffer(), intel_init_bsd_ring_buffer
for ring initialization.

Signed-off-by: Xiang, Haihao <haihao.xiang@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

diff --cc drivers/gpu/drm/i915/i915_gem.c
Simple merge

diff --cc drivers/gpu/drm/i915/intel_ringbuffer.c
Simple merge

diff --cc drivers/gpu/drm/i915/intel_ringbuffer.h
Simple merge