drm/radeon/kms: Forbid creation of framebuffer with no valid GEM object

This will avoid oops if at later point the fb is use. Trying to create
a framebuffer with no valid GEM object is bogus and should be forbidden
as this patch does.

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