drm/i915: Compartmentalize i915_gem_suspend/restore_gtt_mappings

Having made start to better code compartmentalization by introducing
struct intel_gt, continue the theme elsewhere in code by making functions
take parameters take what logically makes most sense for them instead of
the global struct drm_i915_private.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190621070811.7006-22-tvrtko.ursulin@linux.intel.com