drm/i915: Remove unused variable ret

This patch fixes below warnings reported by coccicheck

./drivers/gpu/drm/i915/i915_debugfs.c:789:5-8: Unneeded variable: "ret". Return "0" on line 1012

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zou Wei <zou_wei@huawei.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1603937925-53176-1-git-send-email-zou_wei@huawei.com