dma-buf: remove redundant initialization of sg_table

sg_table is being initialized and is never read before it is updated
again later on, hence making the initialization redundant. Remove
the initialization.

Detected by clang scan-build:
"warning: Value stored to 'sg_table' during its initialization is
never read"

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20170914230516.6056-1-colin.king@canonical.com