msm: kgsl: Avoid dereferencing NULL pointers

It is possible to drop to the bottom of adreno_ringbuffer_issuecmds
with either context or drawctxt set to NULL. Avoid dereferencing these
variables in the trace call at the bottom of the function.

Change-Id: Ic0dedbadbc7e2c0c4ef85e0d7103e04485ed3e69
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>