loader: use HAVE_LIBDRM instead of ! __NOT_HAVE_DRM_H

Double negatives in English language are normally avoided, plus the
former seems cleaner and more consistent.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/loader/SConscript

@@ -8,8 +8,6 @@ env.Prepend(CPPPATH = [
     '#include'
 ])
 
-env.Append(CPPDEFINES = ['__NOT_HAVE_DRM_H'])
-
 if env['udev']:
     env.PkgUseModules('UDEV')
     env.Append(CPPDEFINES = ['HAVE_LIBUDEV'])