drm/i915: move more code to __i915_drm_thaw

Both callers had code to sanitize the uncore and restore the GTT
mappings just before calling __i915_drm_thaw, so Chris suggested I
should unify the code.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>