drm: dumb scanout create/mmap for intel/radeon (v3)

This is just an idea that might or might not be a good idea,
it basically adds two ioctls to create a dumb and map a dumb buffer
suitable for scanout. The handle can be passed to the KMS ioctls to create
a framebuffer.

It looks to me like it would be useful in the following cases:
a) in development drivers - we can always provide a shadowfb fallback.
b) libkms users - we can clean up libkms a lot and avoid linking
to libdrm_*.
c) plymouth via libkms is a lot easier.

Userspace bits would be just calls + mmaps. We could probably
mark these handles somehow as not being suitable for acceleartion
so as top stop people who are dumber than dumb.

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

drivers/gpu/drm/drm_gem.c

@@ -181,7 +181,7 @@ EXPORT_SYMBOL(drm_gem_object_alloc);
 /**
  * Removes the mapping from handle to filp for this object.
  */
-static int
+int
 drm_gem_handle_delete(struct drm_file *filp, u32 handle)
 {
 	struct drm_device *dev;
@@ -214,6 +214,7 @@ drm_gem_handle_delete(struct drm_file *filp, u32 handle)
 
 	return 0;
 }
+EXPORT_SYMBOL(drm_gem_handle_delete);
 
 /**
  * Create a handle for this object. This adds a handle reference