drm/i915/dmabuf: dmabuf cleanup

Some minor cleanup of some variables for consistency.

Normalize struct sg_table to sgt.
Normalize struct dma_buf_attachment to attach.
checkpatch issues sizeof(), !NULL updates.

Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221028155029.494736-3-matthew.auld@intel.com