drm/i915/userptr: Acquire the page lock around set_page_dirty()

set_page_dirty says:

	For pages with a mapping this should be done under the page lock
	for the benefit of asynchronous memory errors who prefer a
	consistent dirty state. This rule can be broken in some special
	cases, but should be better not to.

Under those rules, it is only safe for us to use the plain set_page_dirty
calls for shmemfs/anonymous memory. Userptr may be used with real
mappings and so needs to use the locked version (set_page_dirty_lock).

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=203317
Fixes: 5cc9ed4b ("drm/i915: Introduce mapping of user pages into video memory (userptr) ioctl")
References: 6dcc693b

 ("ext4: warn when page is dirtied without buffers")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: stable@vger.kernel.org
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190708140327.26825-1-chris@chris-wilson.co.uk
(cherry picked from commit cb6d7c7d

)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>