drm/i915/guc: Cache the client mapping

Use i915_gem_object_pin_map() for the guc client's lifetime to replace
the peristent kmap + frequent kmap_atomic with a permanent vmapping.
This avoids taking the obj->mm.lock mutex whilst inside irq context
later.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=98571
Fixes: 96d77634

 ("drm/i915: Use a radixtree for random access...");
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20161102175051.29163-9-chris@chris-wilson.co.uk
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>