msm: kgsl: Check that the adreno drawctxt is valid before destroying it

If there is a failure in drawctxt create, then the KGSL core will call
drawctxt destroy to free the memory.  If the failure happened soon
enough, there might not a drawctxt to free.

CRs-Fixed: 371852
Change-Id: Ic0dedbad629090ae1827e354b9b94bbd3c5efcdd
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>