msm: kgsl: Adds NULL check for drawctxt ops

There is a code path which leaves drawctxt->ops uninitialized and
and calls adreno_drawctxt_detach. Adds a check to make sure we
do not dereference a NULL pointer.

Change-Id: I9d1aa5deb0addeb3eea6d375f197669ccd89cfd4
Signed-off-by: Shrenuj Bansal <shrenujb@codeaurora.org>