drm/i915: Report an error when i915.reset prevents a reset

If the user disables the GPU reset using the i915.reset parameter and
one occurs, report that we failed to reset the GPU. If we return early,
as we currently do, then we leave all state intact (with a hung GPU)
and clients block forever waiting for their requests to complete.

Testcase: igt/gem_eio
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
[danvet: Mark i915.reset as an unsafe modoption, as discussed with
Chris.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>