drm/i915/fbdev: fixup setting screen_size

Since we are actually mapping the object and not the vma, when dealing
with LMEM, we should be careful and use the backing store size here,
since the vma->node.size could have all kinds of funny padding
constraints, which could result in us writing to OOB address.

v2(Chris):
  - Prefer vma->size here, which should be the backing store size. Some
    more rework is needed here to stop using node.size in some other
    places.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220304095934.925036-1-matthew.auld@intel.com