drm/i915/gtt: allow overriding the pt alignment

On some platforms we have alignment restrictions when accessing LMEM
from the GTT. In the next few patches we need to be able to modify the
page-tables directly via the GTT itself.

Suggested-by: Ramalingam C <ramalingam.c@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Ramalingam C <ramalingam.c@intel.com>
Reviewed-by: Ramalingam C <ramalingam.c@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220218184752.7524-10-ramalingam.c@intel.com