msm: kgsl: Add NULL check for kgsl_midframe before accessing it

in kgsl_pwrscale_init(), add missing NULL check for kgsl_midframe
to avoid further accessing, in case if memory is not available.

Change-Id: If6a4e59d4675fe67aefc63d7f8251f4d28ddeec5
Signed-off-by: Rajesh Kemisetti <rajeshk@codeaurora.org>