drm: Make a shared implementation of the dumb mmap offset ioctl

This is basically the CMA implementation, plus calling create_offset
(which cma chooses to do at object creation time).  Many drivers
couldn't be converted, because their mmap offset vma node is in the
ttm bo instead.  Also, armada's implementation checks that the bo
you're trying to map wasn't imported from elsewhere.

Signed-off-by: Eric Anholt <eric@anholt.net>
drm: Make a shared stub encoder implementation.

Signed-off-by: Eric Anholt <eric@anholt.net>
drm: Drop common arguments to drm_fbdev_cma_init().

Both callers were passing the same arguments, so I decided to delete
them rather than adding the same set in another caller.

Signed-off-by: Eric Anholt <eric@anholt.net>
drm/vc4: skeleton "read registers into debugfs" driver.

Signed-off-by: Eric Anholt <eric@anholt.net>