[Intel-gfx] [PATCH] drm/i915: fix build for DEBUG_FS=n

[shuang.he at intel.com]

Tested-By: Intel Graphics QA PRTS (Patch Regression Test System Contact: shuang.he at intel.com)
Task id: 6175
-------------------------------------Summary-------------------------------------
Platform          Delta          drm-intel-nightly          Series Applied
PNV                 -6              276/276              270/276
ILK                                  301/301              301/301
SNB                                  316/316              316/316
IVB                                  328/328              328/328
BYT                                  285/285              285/285
HSW                                  394/394              394/394
BDW                                  321/321              321/321
-------------------------------------Detailed-------------------------------------
Platform  Test                                drm-intel-nightly          Series Applied
 PNV  igt at gem_userptr_blits@coherency-sync      CRASH(1)PASS(2)      CRASH(2)
 PNV  igt at gem_userptr_blits@coherency-unsync      CRASH(1)PASS(3)      CRASH(2)
 PNV  igt at gen3_render_linear_blits      FAIL(3)PASS(2)      FAIL(2)
 PNV  igt at gen3_render_mixed_blits      FAIL(2)PASS(2)      FAIL(1)PASS(1)
 PNV  igt at gen3_render_tiledx_blits      FAIL(3)PASS(2)      FAIL(1)PASS(1)
 PNV  igt at gen3_render_tiledy_blits      FAIL(3)PASS(2)      FAIL(1)PASS(1)
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