drm/i915: Piggy back hangstats off of contexts

To simplify the codepaths somewhat, we can simply always create a
context. Contexts already keep hangstat information. This prevents us
from having to differentiate at other parts in the code.

There is allocation overhead, but it should not be measurable.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>