dma-buf: Fix a few typos in dma-buf documentation

Fix a few typos in the documentation:
- Remove an extraneous 'or'
- 'unpins' -> 'unpin'
- 'braket' -> 'bracket'
- 'mappinsg' -> 'mappings'
- 'fullfills' -> 'fulfills'

Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Gal Pressman <galpress@amazon.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210809122247.15869-1-galpress@amazon.com