drm/vmwgfx: Use preprocessor macro to get valid context node

Several command verifier function check if context node is present or
not and if not present print an error and return. Use a preprocessor
macro to print the message.

v2: Name-space distinction for preprocessor macro

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