drm/vmwgfx: Use memdup_user() rather than duplicating its implementation

* Reuse existing functionality from memdup_user() instead of keeping
  duplicate source code.

* Try this copy operation before allocating memory for the data structure
  member "offsets".

* Delete the local variable "user_sizes" which became unnecessary
  with this refactoring.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>
Signed-off-by: Sinclair Yeh <syeh@vmware.com>