msm: kgsl: Check the return value of _kgsl_context_get()

Check the value returned by _kgsl_context_get() and fail if
appropriate.  This prevents us from accidently increasing the
ref count on a destroyed context.

Change-Id: Ic0dedbad891842a73b1b87eb6671f9a39a275dd4
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>