drm/fb-helper: use gem_bo.resv, not dma_buf.resv in prepare_fb

With

commit 5f6ed987
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Fri Jun 14 22:35:57 2019 +0200

    drm/prime: automatically set gem_obj->resv on import

we consistently set drm_gem_bo.resv for imported buffers. Which means
we don't need to check the dma-buf in the prepare_fb helper, but can
generalize them so they're also useful for display+render drivers
which use gem_bo.resv to track their own rendering for their own
scanout buffers.

Cc: Emil Velikov <emil.velikov@collabora.com>
Cc: Eric Anholt <eric@anholt.net>
Cc: Rob Clark <robdclark@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190625204208.5614-2-daniel.vetter@ffwll.ch