drm/vmwgfx: Implement an infrastructure for write-coherent resources

drm/vmwgfx: Implement an infrastructure for write-coherent resources

This infrastructure will, for coherent resources, make sure that
from the user-space point of view, data written by the CPU is immediately
automatically available to the GPU at resource validation time.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Deepak Rawat <drawat@vmware.com>