drm/msm/a6xx: Remove logically deadcode in a6xx_preempt.c

drm/msm/a6xx: Remove logically deadcode in a6xx_preempt.c

The ternary operator never returns -1 as `ring` will never be NULL.
Thus, the ternary operator is not needed.
Fix this by removing the ternary operation and only including the
value it will return when the `ring` is not NULL.

This was reported by Coverity Scan.
https://scan7.scan.coverity.com/#/project-view/51525/11354?selectedIssue=1600286

Fixes: 35d36dc1692f ("drm/msm/a6xx: Add traces for preemption")
Signed-off-by: Everest K.C. <everestkc@everestkc.com.np>
Patchwork: https://patchwork.freedesktop.org/patch/619349/
Signed-off-by: Rob Clark <robdclark@chromium.org>