drm/i915: Use more optimal VTd alignment for planes

Depending on the platform and/or plane type we can get away
with a bit less alignment in the VT-d w/a. Reduce the numbers
accordingly.

Note that it's not actually clear in VLV/CHV need this w/a,
and if they do we don't actually know what kind of alignment
is sufficient. Leave the 256k alignment in place for now, but
toss in a FIXME.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250122151755.6928-3-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>