drm/vc4: Use dma_resv locking wrappers

I'll add more fancy logic to them soon, so everyone really has to use
them. Plus they already provide some nice additional debug
infrastructure on top of direct ww_mutex usage for the fences tracked
by dma_resv.

Reviewed-by: Eric Anholt <eric@anholt.net>
Cc: Eric Anholt <eric@anholt.net>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191125094356.161941-5-daniel.vetter@ffwll.ch