drm/vmwgfx: Improve on host message error messages

Make sure the error messages are a bit more descriptive, so that
a log reader may understand what's gone wrong.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>