drm/i915: Fix inconsistent naming of i915_guc_client parameter

We usually use 'client' as identifier for the i915_guc_client.
For unknown reason, few functions were using 'gc' name.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
[joonas.lahtinen@linux.intel.com: Split two lines over 80]
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161215195321.63804-1-michal.wajdeczko@intel.com