drm/i915: Don't leak command parser tables on suspend/resume

Ring init and cleanup are not balanced because we re-init the rings on
resume without having cleaned them up on suspend. This leads to the
driver leaking the parser's hash tables with a kmemleak signature such
as this:

unreferenced object 0xffff880405960980 (size 32):
  comm "systemd-udevd", pid 516, jiffies 4294896961 (age 10202.044s)
  hex dump (first 32 bytes):
    d0 85 46 c0 ff ff ff ff 00 00 00 00 00 00 00 00  ..F.............
    98 60 28 04 04 88 ff ff 00 00 00 00 00 00 00 00  .`(.............
  backtrace:
    [<ffffffff81816f9e>] kmemleak_alloc+0x4e/0xb0
    [<ffffffff811fa678>] kmem_cache_alloc_trace+0x168/0x2f0
    [<ffffffffc03e20a5>] i915_cmd_parser_init_ring+0x2a5/0x3e0 [i915]
    [<ffffffffc04088a2>] intel_init_ring_buffer+0x202/0x470 [i915]
    [<ffffffffc040c998>] intel_init_vebox_ring_buffer+0x1e8/0x2b0 [i915]
    [<ffffffffc03eff59>] i915_gem_init_hw+0x2f9/0x3a0 [i915]
    [<ffffffffc03f0057>] i915_gem_init+0x57/0x1d0 [i915]
    [<ffffffffc045e26a>] i915_driver_load+0xc0a/0x10e0 [i915]
    [<ffffffffc02e0d5d>] drm_dev_register+0xad/0x100 [drm]
    [<ffffffffc02e3b9f>] drm_get_pci_dev+0x8f/0x200 [drm]
    [<ffffffffc03c934b>] i915_pci_probe+0x3b/0x60 [i915]
    [<ffffffff81436725>] local_pci_probe+0x45/0xa0
    [<ffffffff81437a69>] pci_device_probe+0xd9/0x130
    [<ffffffff81524f4d>] driver_probe_device+0x12d/0x3e0
    [<ffffffff815252d3>] __driver_attach+0x93/0xa0
    [<ffffffff81522e1b>] bus_for_each_dev+0x6b/0xb0

This patch extends the current convention of checking whether a
resource is already allocated before allocating it during ring init.
Longer term it might make sense to only init the rings once.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=83794


Tested-by: Kari Suvanto <kari.tj.suvanto@gmail.com>
Signed-off-by: Brad Volkin <bradley.d.volkin@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: stable@vger.kernel.org
Signed-off-by: Jani Nikula <jani.nikula@intel.com>