drm/armada: fix page_flip refcounting leak

A refcounting leak was found of the original frame buffer attached to
the CRTC when using the page_flip ioctl, resulting in the frame buffer
never being freed.

This was not obvious initially, as if the page flip subsequently
re-attaches the original frame buffer, the refcounts will be balanced.
However, if the original frame buffer is freed, then it will be leaked.

Fix this by ensuring that we take a reference on the incoming fb, but
rely on the queued work to drop that ref count.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>