drm/i915: gracefully error out on platform with small-bar

Currently we just fatally crash during module load if we encounter
small-BAR configuration on DG2. We have most of the support already, but
we lack the final uAPI bits to tie it all together.

Signed-off-by: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
[mauld: reword the commit and error message slightly]
Link: https://patchwork.freedesktop.org/patch/msgid/20220511153746.14142-3-nirmoy.das@intel.com