drm/i915: Make default value for i915.mmio_debug a compile time option

The normal behaviour is to periodically check for a mmio access error,
and once detected enable mmio access checking. However this is useless
if the error only occurs once during module load, and so we may miss
such errors in CI. To allow ourselves to catch them, allow CI to opt into
always enabling mmio debugging.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190530121311.6794-1-chris@chris-wilson.co.uk