drm/i915: don't use a temp buffer for opregion debugfs file

Hasn't been necessary since

commit 115719fc
Author: Williams, Dan J <dan.j.williams@intel.com>
Date:   Mon Oct 12 21:12:57 2015 +0000

    i915: switch from acpi_os_ioremap to memremap

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/74664a556a56d0eceb0029bbd77ffc1d771b0628.1450089383.git.jani.nikula@intel.com