drm/msm/a6xx: No zap shader is not an error

Depending on platform firmware, a zap shader may not be required to take
the GPU out of secure mode on boot, in which case we can just write
RBBM_SECVID_TRUST_CNTL directly.  Which we *mostly* handled, but missed
clearing 'ret' resulting that hw_init() returned an error on these
devices.

Fixes: abccb9fe

 ("drm/msm/a6xx: Add zap shader load")
Acked-by: Jordan Crouse <jcrouse@codeaurora.org>
Reviewed-by: Sean Paul <sean@poorly.run>
Signed-off-by: Rob Clark <robdclark@chromium.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190508130726.27557-1-robdclark@gmail.com