drm_hwcomposer: remove NVIDIA importer

There's no opensource implementation for the NVIDIA gralloc implementation,
so remove it as it is not testable.

As all of the gralloc perform() operations are specific to it, they can be
removed, too.

Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Robert Foss <robert.foss@collabora.com>
3 files changed