drm: Expose a method for creating anonymous struct file around drm_minor

Sometimes we need to create a struct file to wrap a drm_device, as it
the user were to have opened /dev/dri/card0 but to do so anonymously
(i.e. for internal use). Provide a utility method to create a struct
file with the drm_device->driver.fops, that wrap the drm_device.

v2: Restrict usage to selftests

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20191107180601.30815-2-chris@chris-wilson.co.uk