drm/i915/guc: Move intel_guc_reserved_gtt_size to intel_wopcm_guc_size

Reduces pointer chasing and gets more to the point.

v2:
 * Tidy whitespace.
 * Tidy comment. (Michal)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Suggested-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190611110044.7742-1-tvrtko.ursulin@linux.intel.com