msm: kgsl: Return something when the KGSL device is read

'cat /dev/kgsl-3d0' is the first smoke test that everybody runs when
bringing up a new GPU since it can be run without any other tools
and verifies the full power / initialization path.  Instead of just
returning -EINVAL for a read print a little message to let the user
know that indeed, the GPU is up and happy.

Change-Id: Ic0dedbadf9c8a2a926142a79802d8a340892bc3d
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>