drm/radeon/kms: Do not unpin buffer in fb destruction

It's not necessary to unpin buffer in fb destruction. pin/unpin
need to be balanced and we don't pin in fb creation. We pin when
an fb is associated to a crtc and unpin when the fb is disassociated
from the crtc.

Note:
Maybe we should take reference on fb in set_base callback so fb
doesn't disappear until it's unbind from ctrc.

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>