drm/i915: Make GuC GGTT reservation work on ggtt

These functions operate on ggtt so make them take that directly as
parameter.

At the same time move the USES_GUC conditional down to
intel_guc_reserve_ggtt_top for symmetry with
intel_guc_reserved_gtt_size.

v2:
 * Rename and move functions to be static in i915_gem_gtt.c (Michal)

v3:
 * Add comment explaining reason for reservation, add assert and fix
   error message. (Michal)

v4:
 * Fix checkpatch error.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190611122350.15060-1-tvrtko.ursulin@linux.intel.com