Apply depth even when surface is null

We need to update mDepth even when the surface is null, otherwise
events will be ignored and mDepth will have the wrong value when
waking up from screen-off.

Test: pull up app drawer, screen off, unlock
Test: go to overview, screen off, unlock
Test: launch app, observe blurs and zoom
Fixes: 191153501
Change-Id: I33f5d84a50e24f05a09769b1f7f3c27969f847cd