drm/i915: Record pid/comm of hanging task

After finding the guilty batch and request, we can use it to find the
process that submitted the batch and then add the culprit into the error
state.

This is a slightly different approach from Ben's in that instead of
adding the extra information into the struct i915_hw_context, we use the
information already captured in struct drm_file which is then referenced
from the request.

v2: Also capture the workaround buffer for gen2, so that we can compare
    its contents against the intended batch for the active request.

v3: Rebase (Mika)
v4: Check for null context (Chris)
    checkpatch warnings fixed

Link: http://lists.freedesktop.org/archives/intel-gfx/2013-August/032280.html


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk> (v2)
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com> (v4)
Acked-by: Ben Widawsky <ben@bwidawsk.net>
Cc: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>