drm/i915: Disable all outputs early, before KMS takeover

If the outputs are active and continuing to access the GATT when we
teardown the PTEs, then there is a potential for us to hang the GPU.
The hang tends to be a PGTBL_ER with either an invalid host access or
an invalid display plane fetch.

v2: Reorder IRQ initialisation to defer until after GEM is setup.

Reported-by: Pekka Enberg <penberg@kernel.org>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Tested-by: Daniel Vetter <daniel.vetter@ffwll.ch> (855GM)
Tested-by: Pekka Enberg <penberg@kernel.org>
           # note that this doesn't fix the underlying problem of the
             PGTBL_ER and pipe underruns being reported immediately upon
             init on his 965GM MacBook
Reported-and-tested-by: Rick Bramley <richard.bramley@hp.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=35635


Reported-and-tested-by: Zdenek Kabelac <zdenek.kabelac@gmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=36048


Reviewed-by: Ben Widawsky <ben@bwidawsk.net>