[media] vsp1: use proper dma alloc/free functions

I noticed this while merging the drm tree and checking for stragglers:
the vsp1 driver still used dma_[alloc|free]_writecombine() that got
renamed in commit f6e45661f9be ("dma, mm/pat: Rename
dma_*_writecombine() to dma_*_wc()")

I should have noticed back in the media merge (commit bace3db5da97), but
better late than never.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>