drm/i915: Identify active VM for batchbuffer capture

Using the current state of the page directory registers, we can
determine which of our address spaces was active when the hang occurred.
This allows us to scan through all the address spaces to identify the
"active" one during error capture.

v2: Rebased for BDW error detection. BDW error detection is similar
except instead of PP_DIR_BASE, we can use the PDP registers.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
[danvet: Add FIXME about global gtt misuse.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>