drm/etnaviv: fix mmap operations for userptr and dma-buf objects

Add an indirect object operations call to allow distinct implementations
of the mmap operation based on the type of the object.

This ensures that the exporter is called to set up the mmap for imported
dma-bufs and disallows mapping of userptr objects through the DRM file,
as this might lead to serious corruption of kernel internal state.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>