drm/i915: Check we have an wake device before flushing GTT writes

drm/i915: Check we have an wake device before flushing GTT writes

We can assume that if the device is asleep then all pending GTT writes
will have been posted, and so we can defer the flush from
i915_gem_object_flush_gtt_write_domain()

[ 1957.462568] WARNING: CPU: 0 PID: 6132 at drivers/gpu/drm/i915/intel_drv.h:1742 fwtable_read32+0x123/0x150 [i915]
[ 1957.462582] RPM wakelock ref not held during HW access
[ 1957.462583] Modules linked in: i915 intel_gtt drm_kms_helper prime_numbers
[ 1957.462607] CPU: 0 PID: 6132 Comm: gem_concurrent_ Tainted: G     U          4.11.0-rc1+ #464
[ 1957.462619] Hardware name:                  /        , BIOS PYBSWCEL.86A.0027.2015.0507.1758 05/07/2015
[ 1957.462630] Call Trace:
[ 1957.462646]  dump_stack+0x4d/0x6f
[ 1957.462657]  __warn+0xc1/0xe0
[ 1957.462667]  warn_slowpath_fmt+0x4a/0x50
[ 1957.462709]  fwtable_read32+0x123/0x150 [i915]
[ 1957.462750]  i915_gem_object_flush_gtt_write_domain+0x43/0x70 [i915]
[ 1957.462791]  i915_gem_object_set_to_cpu_domain+0x46/0xa0 [i915]
[ 1957.462831]  i915_gem_set_domain_ioctl+0x15d/0x220 [i915]
[ 1957.462843]  drm_ioctl+0x1d7/0x440
[ 1957.462885]  ? i915_gem_obj_prepare_shmem_write+0x1d0/0x1d0 [i915]
[ 1957.462896]  ? pick_next_task_fair+0x436/0x440
[ 1957.462906]  ? mntput+0x1f/0x30
[ 1957.462915]  do_vfs_ioctl+0x8f/0x5c0
[ 1957.462925]  ? __schedule+0x16f/0x5f0
[ 1957.462935]  ? ____fput+0x9/0x10
[ 1957.462943]  SyS_ioctl+0x3c/0x70
[ 1957.462952]  entry_SYSCALL_64_fastpath+0x17/0x98
[ 1957.462961] RIP: 0033:0x7fc542179ca7
[ 1957.462968] RSP: 002b:00007ffeef12ff98 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
[ 1957.462982] RAX: ffffffffffffffda RBX: 00007ffeef1301d0 RCX: 00007fc542179ca7
[ 1957.462990] RDX: 00007ffeef12ffd0 RSI: 00000000400c645f RDI: 0000000000000003
[ 1957.462999] RBP: 0000000000000003 R08: 000055f433bc7c40 R09: 000000000000002c
[ 1957.463006] R10: 0000000000000073 R11: 0000000000000246 R12: 0000000000000018
[ 1957.463015] R13: 000055f432c89d20 R14: 000055f432c87690 R15: 0000000000000000

Fixes: 3b5724d702ef ("drm/i915: Wait for writes through the GTT to land before reading back")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170323150053.28582-1-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>