drm/i915: Test all device memory on probing

This extends the previous sanitychecking of device memory to read/write
all the memory on the device during the device probe, ala memtest86,
as an optional module parameter: i915.memtest=1. This is not expected to
be fast, but a reasonably thorough verfification that the device memory
is accessible and doesn't return bit errors.

v2: Rebased.

Suggested-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211208153404.27546-4-ramalingam.c@intel.com