drm/i915: include GTT page-size info in error state

It might prove useful in the future to know if the vma is utilising
huge-GTT-pages. Related to this is the GTT cache, where there is some HW
"quirkiness" where it must be disabled if using 2M pages, so include
that for good measure.

Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190909171646.22090-1-matthew.auld@intel.com