qxl: Include only the dpms headers we need
dpms.h is provided by libXext, but nothing in our configure.ac is verifying that we have libXext's headers. Fortunately we only need the definitions in dpmsconst.h (which dpms.h included for us), which is in xorgproto and thus implied by having an xserver DDK to build against.
And we're even trying to include dpmsconst.h when we think we have it, meaning when HAVE_XEXTPROTO_71 is defined, but while many other drivers define that macro in their configure.ac, we for no particularly good reason do not. Oops. But since xextproto is about ten years old by now we can probably just safely include it unconditionally.