dma-buf: dma-buf: stop mapping sg_tables on attach v2

dma-buf: dma-buf: stop mapping sg_tables on attach v2

As a workaround to smoothly transit from static to dynamic DMA-buf
handling we cached the sg_table on attach if dynamic handling mismatched
between exporter and importer.

Since Dmitry and Thomas cleaned that up and also documented the lock
handling we can drop this workaround now.

V2: implement Sima's comments

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Simona Vetter <simona.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20250211163109.12200-4-christian.koenig@amd.com