drm/i915: Remove unused "ret" variable.

Just a small clean-up with no functional change, only
removing a variable that is never actually used.

Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by:  <nathan.d.ciobanu@linux.intel.com>
Reviewed-by: Nathan Ciobanu <nathan.d.ciobanu@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180719234217.7855-1-rodrigo.vivi@intel.com