msm: kgsl: Add a check before accessing SPTPRAC head switch

SPTPRAC head switch is not present all A6xx GPUs. Attempt to
turn on SPTPRAC head switch such as A640 GPU will result into
failure. Fix this by adding a conditional check before we
attempt to turn it on/off.

Change-Id: I9e3af0f31e164efe61bb4dd13b7ec3e89e609039
Signed-off-by: Hareesh Gundu <hareeshg@codeaurora.org>