dma-buf: add SPDX header and fix style in dma-resv.c

dma_resv_lockdep() seems to have some space/tab mixups. Fix that and
move the function to the end of the file.

Also fix some minor things checkpatch.pl pointed out while at it.

No functional change.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210602140359.272601-2-christian.koenig@amd.com