drm/radeon/cik: Move 'r600_ih_ring_{alloc, fini}()'s prototypes to shared header

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/radeon/r600.c:3480:5: warning: no previous prototype for ‘r600_ih_ring_alloc’ [-Wmissing-prototypes]
 3480 | int r600_ih_ring_alloc(struct radeon_device *rdev)
 | ^~~~~~~~~~~~~~~~~~
 drivers/gpu/drm/radeon/r600.c:3516:6: warning: no previous prototype for ‘r600_ih_ring_fini’ [-Wmissing-prototypes]
 3516 | void r600_ih_ring_fini(struct radeon_device *rdev)
 | ^~~~~~~~~~~~~~~~~

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Cc: linux-media@vger.kernel.org
Cc: linaro-mm-sig@lists.linaro.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>