drm/vgem: drop DRIVER_PRIME (v2)

For actual sharing of buffers with other drivers (ie. actual hardware)
we'll need to pimp things out a bit better to deal w/ caching, multiple
memory domains, etc.  See thread:

  http://lists.freedesktop.org/archives/dri-devel/2015-May/083160.html



But for the llvmpipe use-case this isn't a problem.  Nor do we really
need prime/dri3 (dri2 is sufficient).  So until the other issues are
sorted lets remove DRIVER_PRIME.

v2: also drop the dead code

[airlied:
 Okay I'm convinced this API could have a lot of use cases
 that are really really bad, yes the upload use case is valid
 however that isn't the only use case enabled, and if we allow
 all the other use cases, people will start to (ab)use them,
 and then they'll be ABI and my life will get worse, so disable
 PRIME for now]

Acked-by: Thomas Hellstrom <thellstrom@vmware.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>