drm/i915: Enable inject_load_failure only in DEBUG config

drm/i915: Enable inject_load_failure only in DEBUG config

We're using i915_inject_load_failure() to inject dummy
faults during driver load, but since this is debug utility
we shouldn't expose it in default config as it consumes
both code and data.

add/remove: 0/1 grow/shrink: 0/2 up/down: 0/-302 (-302)
Function                                     old     new   delta
__i915_inject_load_failure                    61       -     -61
i915_gem_init                               1331    1268     -63
i915_driver_load                            5923    5745    -178
Total: Before=1177454, After=1177152, chg -0.03%

add/remove: 0/1 grow/shrink: 0/0 up/down: 0/-4 (-4)
Data                                         old     new   delta
i915_load_fail_count                           4       -      -4
Total: Before=56762, After=56758, chg -0.01%

add/remove: 4/8 grow/shrink: 0/1 up/down: 245/-591 (-346)
RO Data                                      old     new   delta
__param_str_inject_load_failure               20       -     -20
__UNIQUE_ID_inject_load_failuretype200        34       -     -34
__param_inject_load_failure                   40       -     -40
__func__                                    4998    4896    -102
__UNIQUE_ID_inject_load_failure201           150       -    -150
Total: Before=119095, After=118749, chg -0.29%

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180201173248.3912-1-michal.wajdeczko@intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>