[Intel-gfx] [PATCH 4/4] drm/i915/bdw: Pin the ringbuffer backing

[shuang.he at intel.com]

Tested-By: PRC QA PRTS (Patch Regression Test System Contact: shuang.he at intel.com)
-------------------------------------Summary-------------------------------------
Platform: baseline_drm_intel_nightly_pass_rate->patch_applied_pass_rate
BYT: pass/total=354/354->353/354
PNV: pass/total=331/331->331/331
ILK: pass/total=341/342->338/342
IVB: pass/total=271/271->271/271
SNB: pass/total=271/271->271/271
HSW: pass/total=271/271->271/271
BDW: pass/total=271/271->271/271
-------------------------------------Detailed-------------------------------------
test_platform: test_suite, test_case, result_with_drm_intel_nightly->result_with_patch_applied
BYT: Intel_gpu_tools, igt_gem_bad_reloc_negative-reloc-lut, PASS->NSPT
ILK: Intel_gpu_tools, igt_drv_suspend_debugfs-reader, PASS->DMESG_WARN
ILK: Intel_gpu_tools, igt_kms_flip_wf_vblank-interruptible, PASS->DMESG_WARN
ILK: Intel_gpu_tools, igt_kms_flip_wf_vblank-vs-modeset, PASS->DMESG_WARN