drm/i915/hangcheck: Prevent long walks across full-ppgtt

With full-ppgtt, it takes the GPU an eon to traverse the entire 256PiB
address space, causing a loop to be detected. Under the current scheme,
if ACTHD walks off the end of a batch buffer and into an empty
address space, we "never" detect the hang. If we always increment the
score as the ACTHD is progressing then we will eventually timeout (after
~46.5s (31 * 1.5s) without advancing onto a new batch). To counter act
this, increase the amount we reduce the score for good batches, so that
only a series of almost-bad batches trigger a full reset. DoS detection
suffers slightly but series of long running shader tests will benefit.

Based on a patch from Chris Wilson.

Testcase: igt/drv_hangman/hangcheck-unterminated
Cc: Daniele Ceraolo Spurio <[email protected]>
Cc: Chris Wilson <[email protected]>
Signed-off-by: Mika Kuoppala <[email protected]>
Reviewed-by: Chris Wilson <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]

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