msm: kgsl: Use SET_SECURE_MODE packet to put GPU in unsecure mode

Access to the SECVID_TRUST_CNTL register is protected from HLOS.
Instead of writing to this register, use the SET_SECURE_MODE
packet to bring up the GPU in unsecure mode so that we also
execute the zap shader firmware.

Change-Id: I3ada2e94cd24fc060c742e3fbe44249505e17290
Signed-off-by: Shrenuj Bansal <shrenujb@codeaurora.org>