drm: fb refcounting for dirtyfb_ioctl

We only need to ensure that the fb stays around for long enough. While
at it, only grab the modeset locks when we need them (since most
drivers don't implement the dirty callback, this should help jitter
and stalls when using the generic modeset driver).

Reviewed-by: Rob Clark <rob@ti.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>