drm/i915: Dump the whole context object.

As we've learned over time, the HW context is just a series of GPU
commands that we're able to decode without any changes in
intel_error_decode. Since many bugs recently have been implicated in
the HW context state, it makes sense to dump the whole context object
in a form which can be parsed.

Sample:
render ring --- HW Context = 0x042db000
ringbuffer (render ring) at 0x0160c000; HEAD points to: 0x0160c000
0x0160c000:      0x00000000: MI_NOOP
0x0160c004:      0x00000000: MI_NOOP
0x0160c008:      0x00000000: MI_NOOP
0x0160c00c:      0x00000000: MI_NOOP
0x0160c010:      0x00000000: MI_NOOP
0x0160c014:      0x00000000: MI_NOOP
0x0160c018:      0x00000000: MI_NOOP
0x0160c01c:      0x00000000: MI_NOOP

Unfortunately, our decoder isn't quite smart enough to deal with the
variable length LRIs - but that is a tools problem.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
[danvet: Clarify commit message a bit, seems to have lost a few
crucial words.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>