dma-buf: drop excl_fence parameter from dma_resv_get_fences

Returning the exclusive fence separately is no longer used.

Instead add a write parameter to indicate the use case.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20211207123411.167006-4-christian.koenig@amd.com