drm/msm: update uapi header license

The same file in libdrm is, as is the tradition with the rest of libdrm,
etc, using an MIT license.  To avoid complications in the future with
sync'ing the uapi header to libdrm, lets fix the license mismatch now
before there are any non-trivial commits from someone other than myself.

Cc: Emil Velikov <emil.l.velikov@gmail.com>
Cc: Gabriel Laskar <gabriel@lse.epita.fr>
Cc: Mikko Rapeli <mikko.rapeli@iki.fi>
Signed-off-by: Rob Clark <robdclark@gmail.com>