drm: Make the fb refcount handover less magic

drm: Make the fb refcount handover less magic

Instead of assigning the plane->fb pointer and clearing the fb pointer
to hand over the reference, let's just do it by grabbing another
referece for plane->fb and let fb keep its original one.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180322152313.6561-8-ville.syrjala@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>