drm/i915: align the plane_vma to min_page_size of stolen mem

Align the plane vma size to the stolem memory regions' min_page_size.

Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
cc: Matthew Auld <matthew.auld@intel.com>
cc: Chris P Wilson <chris.p.wilson@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220128185209.18077-3-ramalingam.c@intel.com