drm/vmwgfx: Use preprocessor macro for cmd struct

Use preprocessor macro for repetitive device command struct format.

v2: Name-space distinction for preprocessor macro.

v3: Struct name as macro parameter and rebase.
Signed-off-by: Deepak Rawat <drawat@vmware.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>