drm/i915: Increase GuC log buffer size to reduce flush interrupts

In cases where GuC generate logs at a very high rate, correspondingly
the rate of flush interrupts is also very high.
So far total 8 pages were allocated for storing both ISR & DPC logs.
As per the half-full draining protocol followed by GuC, by doubling
the number of pages, the frequency of flush interrupts can be cut down
to almost half, which then helps in reducing the logging overhead.
So now allocating 8 pages apiece for ISR & DPC logs.
This also helps in reducing the output log file size, apart from
reducing the flush interrupt count. With the original settings,
44 KB was needed for one snapshot. With modified settings, 76 KB is
needed for a snapshot which will be equivalent to 2 snapshots of the
original setting. So 12KB saving, every 88 KB, over the original setting.

Suggested-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Akash Goel <akash.goel@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>