drm/i915: Sanitycheck device iomem on probe

As we setup the memory regions for the device, give each a quick test to
verify that we can read and write to the full iomem range. This ensures
that our physical addressing for the device's memory is correct, and
some reassurance that the memory is functional.

v2: wrapper for memtest [Chris]

v3: Removed the unused ptr i915 [Chris]

v4: used the %pa for the resource_size_t.

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/20211209162620.5218-1-ramalingam.c@intel.com