drm/i915/selftests: Take runtime wakeref for igt_ggtt_lowlevel

Being a "low-level" test, we opt to bypass the normal bind/unbind hooks
for the lower level insert_entries/clear_range. For ggtt, the
bind/unbind hooks provide the runtime wakeref and so we must also handle
this in exercising the low level hooks.

<4> [538.151672] RPM raw-wakeref not held
<4> [538.151825] WARNING: CPU: 0 PID: 11 at ./drivers/gpu/drm/i915/intel_runtime_pm.h:107 fwtable_read32+0x1be/0x300 [i915]
<4> [538.151830] Modules linked in: i915(+) amdgpu gpu_sched ttm vgem snd_hda_codec_hdmi snd_hda_codec_realtek snd_hda_codec_generic mei_hdcp btusb btrtl btbcm x86_pkg_temp_thermal coretemp btintel crct10dif_pclmul bluetooth crc32_pclmul snd_intel_nhlt snd_hda_codec ecdh_generic ghash_clmulni_intel ecc snd_hwdep snd_hda_core lpc_ich r8169 realtek snd_pcm mei_me mei prime_numbers pinctrl_broxton pinctrl_intel [last unloaded: i915]
<4> [538.151861] CPU: 0 PID: 11 Comm: migration/0 Tainted: G     U            5.3.0-rc7-CI-Trybot_4938+ #1
<4> [538.151864] Hardware name: Intel corporation NUC6CAYS/NUC6CAYB, BIOS AYAPLCEL.86A.0056.2018.0926.1100 09/26/2018
<4> [538.151960] RIP: 0010:fwtable_read32+0x1be/0x300 [i915]
<4> [538.151965] Code: e8 e7 f9 5f e0 e9 0b ff ff ff 80 3d d5 8d 26 00 00 0f 85 81 fe ff ff 48 c7 c7 ef 01 bd a0 c6 05 c1 8d 26 00 01 e8 b2 e4 6a e0 <0f> 0b e9 67 fe ff ff 80 3d ad 8d 26 00 00 0f 85 65 fe ff ff 48 c7
<4> [538.151969] RSP: 0018:ffffc9000007be10 EFLAGS: 00010086
<4> [538.151972] RAX: 0000000000000000 RBX: ffff88826be10d50 RCX: 0000000000000002
<4> [538.151975] RDX: 0000000080000002 RSI: 0000000000000000 RDI: 00000000ffffffff
<4> [538.151978] RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
<4> [538.151981] R10: 0000000000000000 R11: ffffc9000007bcb0 R12: 0000000000101008
<4> [538.151984] R13: 0000000000000000 R14: ffffc9000036f638 R15: 0000000000000002
<4> [538.151987] FS:  0000000000000000(0000) GS:ffff888277a00000(0000) knlGS:0000000000000000
<4> [538.151990] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
<4> [538.151993] CR2: 00007fd48e7052f8 CR3: 0000000005210000 CR4: 00000000003406f0
<4> [538.151995] Call Trace:
<4> [538.152106]  bxt_vtd_ggtt_clear_range__cb+0x38/0x40 [i915]

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190909110011.8958-2-chris@chris-wilson.co.uk